U8RA 


RARJ 


THE 

MUSCLES  OF  THE  EYE 


BY 

LUCIENlHOWE,  M.A.,  M.D. 

PROFESSOR  OF  OPHTHALMOLOGY,  UNIVERSITY  OF  BUFFALO 
MEMBER  OF  THE  ROYAL  COLLEGE  OF  SURGEONS  OF  ENGLAND 

MEMBER  OF  THE  OPHTHALMOLOGISCHE   GESELLSCHAFT,   OF  THE  SOCIETE  FRANfAISE 
D'OPHTALMOLOGIE,  AND  OF  THE  OPHTHALMOLOGICAL  SOCIETY 

OF  THE  UNITED   KINGDOM 
FORMER  PRESIDENT,  SEC.  OPHTHALMOLOGY,  AMERICAN  MEDICAL  ASSOCIATION 


IN  TWO  VOLUMES 


VOLUME    I. 
ANATOMY   AND    PHYSIOLOGY 

INCLUDING 

INSTRUMENTS    FOR    TESTING 

AND 

METHODS   OF  MEASUREMENT 


ILLUSTRATED 


G.  P.  PUTNAM'S  SONS 
NEW  YORK  AND  LONDON 

Cbe  "Knickerbocker  press 
1907 


COPYRIGHT,  1906 

BY 

LUCIEN  HOWE 


PREFACE. 

THE  importance  of  this  subject  becomes  apparent  when 
it  is  remembered  that  abnormal  conditions  of  the  ocular 
muscles — including  those  involved  in  accommodation — con- 
stitute, for  most  ophthalmologists,  by  far  the  largest  group 
of  cases  presented  for  treatment.  Opticians  also  have  to 
deal  constantly  with  questions  in  which  the  ciliary  muscle 
is  a  factor,  and  even  if  they  limit  their  work  to  giving  glasses 
for  presbyopia  and  hypermetropia,  it  is  desirable  that  it  be 
done  intelligently.  All  physicians  are  confronted  at  times 
by  the  numerous  reflexes  associated  with  faulty  action  of 
the  ocular  muscles,  while  their  partial  or  total  paralyses 
are  important  guides  for  the  neurologists  in  diagnosis  and 
treatment. 

And  yet  the  widely  differing  views  concerning  many 
fundamental  facts  and  principles  indicate  that  thus  far  we 
have  succeeded  only  in  accumulating  a  considerable  amount 
of  ignorance  in  regard  to  this  subject.  Indeed,  there  is 
hardly  any  branch  of  medicine  about  which  so  much  has 
been  written,  and  of  which  we  know  in  proportion  so  little. 

It  would,  therefore,  be  worse  than  useless  to  add  to  this 
mass  of  literature  were  it  not  on  a  plan  different  in  its  scope 
from  that  of  any  of  the  detached  articles  or  text-book 
chapters  on  the  subject. 

The  objects  of  this  study  are  : 

First.  To  collect  data  relating  to  this  subject,  separating 
as  clearly  as  possible  demonstrated  facts  from  statements 
based  on  theory. 

Second.  To  formulate  these  facts  concisely,  and  in  the 
simplest  terms  possible. 

In  doing  so  it  is  necessary  to  retain  a  few  mathematical 
statements,  either  because  they  can  not  be  found  elsewhere, 
or  to  show  the  basis  of  other  facts.  These  portions,  how- 


iv  Preface 

ever,  are  made  as  brief  as  possible ;  they  are  set  in  small 
type,  and  cover  altogether  about  a  dozen  pages. 

The  third  reason  for  this  study  is  the  desire  to  supply 
at  least  a  few  of  the  data  which  are  needed  to  correlate 
our  anatomical  and  physiological  facts  with  our  clinical 
experiences. 

The  book,  therefore,  is  in  certain  portions  a  digest  of 
studies  in  which  the  writer  has  been  interested  for  several 
years.  Among  the  results  of  these  may  be  mentioned  the 
distinction  shown  between  the  primary  and  secondary  in- 
sertions, the  illustrations  of  muscular  insertions  by  photo- 
graphs, a  simplified  method  of  recognizing  the  malposition 
of  the  lens  with  the  ophthahnometer,  the  clinical  importance 
of  the  accessory  muscles  of  accommodation,  another  ophthal- 
motrope,  the  measurement  of  the  lifting  power  of  the  ad- 
ductors, the  clinical  measurement  by  photography  of  the 
rate  of  the  lateral  movements,  the  distinction  between  the 
actual  and  apparent  static  position,  between  the  minimum 
and  maximum  dynamic  conditions,  and  the  most  complete 
statement  yet  made  of  the  measurements  of  relative  accom- 
modation, convergence,  and  torsion. 

While  this  volume  is  intended  as  a  statement,  brief  and 
imperfect  though  it  be,  of  what  we  know  of  this  subject 
at  the  present  time,  an  attempt  is  also  made  to  have  it 
useful  otherwise  to  future  students,  and  for  this  purpose 
several  appendices  have  been  added. 

In  order  to  make  each  volume  as  complete  as  possible 
the  questions  and  references  which  relate  to  anatomy  and 
physiology  are  given  in  this  one,  and  the  remaining  parts 
of  these  two  appendices  will  follow  in  the  second"  volume. 

Next  to  the  effort  to  secure  exactness  and  simplicity,  the 
aim  of  the  writer  has  been  to  keep  constantly  in  mind  the 
practical  aspects  of  the  subject.  Whenever  any  salient 
point  is  reached,  a  halt  is  called  in  order  to  take  bearings 
and  determine  its  relation  to  the  pole-star  of  clinical 
experience. 

Appreciating  fully  that  passages  which  appear  plain 
enough  to  a  writer  are  often  confusing  to  the  reader,  es- 
pecially in  the  treatment  of  a  technical  subject  like  this, 


Preface  v 

the  manuscript,  with  the  exception  of  the  portions  in  small 
type,  was  read  by  a  student  who  had  received  hardly  more 
than  a  high-school  education.  When  he  marked  a  passage 
as  not  entirely  clear,  it  was  rewritten  or  stricken  out.  No 
future  critic  can  be  more  unrelenting  than  he. 

Grateful  acknowledgment  should  be  made  to  Professor 
Kallius  of  Gottingen  for  his  criticism  of  the  description  of 
the  secondary  insertions  of  the  muscles,  to  Professor  Bern- 
heimer  of  Innsbruck  for  his  review  of  the  part  relating  to 
the  nerve  supply,  to  Professor  Hess  of  Wiirzburg  for  his  per- 
sonal demonstration  of  the  action  of  the  lens  and  for  his 
valuable  suggestions  concerning  relative  accommodation, 
and  to  Professor  Tscherning  of  Paris  for  his  careful 
criticism  of  the  first  draft  of  the  sections  on  torsion. 

It  should  be  understood  that  none  of  these  eminent  col- 
leagues is  in  any  way  responsible  for  the  imperfections  of 
the  book,  which  are  so  numerous  and  so  evident  to  the 
author.  But  such  friendly  counsel  helps  a  work  greatly 
in  preparing  it  for  the  searching  criticism  with  which  the 
reviewer  greets  a  newcomer  in  the  literary  field. 

Thanks  should  also  be  expressed  to  Dr.  Edward  Jackson  of 
Denver,  who  has  looked  over  the  arrangement  of  the  whole 
after  it  was  ready  for  the  press,  and  to  Dr.  J.  C.  Clemesha 
of  Buffalo,  who  has  assisted  in  the  correction  of  the  proofs. 
In  spite  of  all  the  attention  given  to  details,  it  is  probable 
that  many  faults  and  omissions  will  be  evident.  But  such 
errors  will  be  carefully  noted  in  order  that  others  of  a 
similar  kind  may  not  appear  in  the  second  volume. 


CONTENTS. 
VOLUME  I. 

ANATOMY  AND  PHYSIOLOGY. 

PART  I. 
ANATOMY. 

CHAPTER  I. 

PAGE 

THE   EXTRAOCULAR  MUSCLES I 

Reason  for  reviewing  the  anatomy — Instruments  for  dissection, 
etc. — Dissections  of  the  orbits  of  animals ;  of  the  human  orbit — 
Inflation  of  the  globe — Lines  of  origin  of  the  muscles — Meaning 
of  ' '  primary  "  and  "  secondary  "  insertions — The  levator  palpebrse — 
Internal  rectus — External  rectus — Superior  rectus — Inferior  rec- 
tus — Superior  oblique — Inferior  oblique — Measurements  of  the 
primary  insertions ;  of  the  secondary  insertions — General  plan 
of  the  arrangement  of  the  connective  tissue  of  the  orbit — 
Fascia  orbito-ocularis  and  the  check  ligaments — Supernumerary 
muscles  of  the  orbit — Influence  of  heredity  on  the  ocular  muscles. 

CHAPTER  II. 

THE   INTRAOCULAR   MUSCLES   AND   OTHER    STRUCTURES    CONCERNED    IN 

ACCOMMODATION 56 

The  ciliary  muscle — Ligament  of  Zinn — Structure  of  the  lens — 
Position  of  the  lens — How  we  can  determine  this — Simple  modifi- 
cation of  the  Javal  ophthalmometer  for  estimating  the  position  of 
the  lens — Ophthalmophacometer  of  Tscherning — Imperfections  of 
the  media  which  influence  the  ciliary  muscle — Clinical  importance 
of  the  refractive  media — Accessory  muscles  of  accommodation. 

CHAPTER  III. 

NERVE   SUPPLY 85 

Macroscopic  anatomy — Motor  oculi — Arrangement  of  the  cells  in 
the  nucleus — Gyrus — Connecting  fibers—  Relation  of  certain  groups 


viii  Contents 

PAGE 

of  cells  of  the  nucleus  to  certain  ocular  muscles — Fourth  nerve — 
Fifth  nerve — Sixth  nerve — Branches  of  the  sympathetic. 

CHAPTER  IV. 
BLOOD-VESSELS IO8 

CHAPTER  V. 

COMPARATIVE  ANATOMY  AND  EMBRYOLOGY  OF  THE  MUSCLES  .  .         IIO 

PART  II. 
PHYSIOLOGY. 

CHAPTER    I. 

ONE  EYE  AT  REST 117 

Introduction — Geometry  of  the  globe — Center  of  motion — Calcula- 
tion of  the  center  of  motion — The  angle  alpha  (delta) — Clinical 
value  of  this  angle — Relation  of  the  visual  acuity  to  the  motion  of 
the  eye — Suppression  of  diplopia  physiological — Monocular  position 
of  rest. 

CHAPTER  II. 

ONE  EYE  IN  ACTION,  BUT  NOT  NECESSARILY  IN  MOTION  (ACCOMMODATION),        136 

Range  of  accommodation — How  a  lens  affects  the  focal  power — 
Pupillary  reaction — Astigmatic  accommodation — How  to  measure 
the  effects  of  cycloplegics  and  myotics — Atropin  in  full  doses — 
Minimum  doses  of  atropin — Clinical  value  of  these  effects — Homa- 
tropin — Comparison  of  atropin  and  homatropin — Cocain — Eserin 
in  full  doses — Eserin  in  minimum  doses — Diagnostic  value  of 
minimum  doses — Clinical  methods  of  measurement. 

CHAPTER  III. 

ONE  EYE  IN  MOTION 179 

Nomenclature — Ophthalmotropes — Action  of  a  single  muscle — 
Movement  in  any  direction  the  resultant  of  two  or  more  actions — 
The  opposing  action  of  muscles — Pre-eminent  and  subsidiary 
motions — Muscles  which  cause  rotation  in  certain  directions — 
Field  of  fixation  and  methods  of  measuring  it — The  tropo- 
meter — Extent  of  the  field  of  fixation — Practical  importance  of 
the  field  of  fixation — Lifting  power  of  the  adductors  and  its  clinical 
value — Tensile  strength  of  the  recti — How  the  rapidity  of  lateral 
movement  is  measured  by  photography — Movement  of  the  eyes 


Contents  ix 

PAGE 

when  reading — Measurement  of  winking  and  its  clinical  value — 
Wheel  motion  with  one  eye — Sound  produced  by  the  muscles. 

CHAPTER  IV. 

BOTH  EYES  AT  REST 214 

DIVISION  I. 

GENERAL  CONSIDERATIONS .         214 

On  the  measurement  of  the  interocular  base  line — Physiological 
heterophoria — Precautions  necessary  with  all  tests — The  room — 
Light — Head-rest — Value  of  foregoing  precautions. 

DIVISION  II. 

TEST  TO  DETERMINE  THE  POSITION  OF  REST  OF  THE  VISUAL  AXES  .  224 
Classification  of  the  tests — First  group  (displacement  of  one  or 
both  retinal  images),  with  single  prism — Phorometers,  etc. — Second 
group  (blurring  of  one  image) — Maddox  rod — Lens — Cobalt  glass, 
etc. — Third  group  (excluding  of  one  image) — Cover  test — Diplo- 
scope — Which  is  the  best  of  these  methods  ? — Influence  of  the  eyes 
in  determining  static  position — What  is  the  usual  position  of  rest  ? 
— Difference  between  the  apparent  and  actual  static  position — 
Conclusions. 

DIVISION  III. 

TESTS  TO   DETERMINE  THE  POSITION   OF   THE   VERTICAL  AXES       .  .         244 

Does  the  globe  revolve  on  its  antero-posterior  axis,  to  the  position 
of  rest  ? — Stevens'  clinoscope — Author's  clinosc9pe — Usual  position 
of  the  vertical  axes — Which  test  is  best  ? — Nature  of  cyclophoria. 

CHAPTER  V. 

BOTH  EYES  IN  MOTION  ;  AND  FIRST  GROUP  OF  ASSOCIATED  MOVEMENTS  .  253 
Definition— Nerve  impulses — Classification — First  group  :  Visual 
axes  in  the  primary  position  with  the  vertical  axes  rotating  about 
them — Hering's  method — Donders'  method — Maximum  and  mini- 
mum intorsion  and  extorsion — Physiological  amount — Clinical  im- 
portance of  this  form  of  torsion. 

CHAPTER  VI. 

SECOND  GROUP  OF  ASSOCIATED  MOVEMENTS  .  .  .  .  .         265 

Parallel  visual  axes  moving  in  the  principal  meridians  (no  torsion). 
CHAPTER  VII. 

THIRD  GROUP  OF  ASSOCIATED  MOVEMENTS       .  .  .  .  .         267 

Parallel  visual  axes  moving  in  oblique  directions  producing  false 
torsion — False  torsion  not  a  true  wheel  motion — Conclusions  from 
experiments  with  the  after-images— Donders'  law — Listing's  law — 


x  Contents 

PAGE 

Calculation  of  false  torsion — Aids  to  calculation — Clinical  value  of 
false  torsion. 

CHAPTER  VIII. 

FOURTH  GROUP  OF  ASSOCIATED  MOVEMENTS 280 

Visual  axes  moving  in  opposite  directions  at  the  same  time  (Con- 
vergence). 

DIVISION  I. 

PRISMS  ............          28O 

Ophthalmological  prisms — Numbering  of  prisms — In  what  forms 
are  prisms  arranged? — Results  produced  by  the  combination  of 
prisms — What  is  the  actual  deflection  produced  by  a  prism  ?  (table) — 
Prismatic  effects  produced  by  decentering  lenses. 

DIVISION  II. 

CONVERGENCE .  .         2Q2 

Definition  of  the  meter  angle — Meter  angles  expressed  in  degrees 
(table) — Degrees  expressed  in  meter  angles  (table) — What  is  the 
relative  or  fusion  power? — How  is  this  measured? — What  is  the 
amount  of  the  minimum  fusion  power? — What  is  the  amount  of 
the  maximum  fusion  power? — Balance  of  power  in  groups  of  muscles 
— Test  of  muscle  balance  with  convergence — Stereoscopes. 

DIVISION  III. 

RELATIVE  ACCOMMODATION 309 

Definition  and  range  of  relative  accommodation — Illustration  of  the 
ranges  of  accommodation  for  varying  degrees  of  convergence — De- 
siderata for  the  accurate  measurement  of  relative  accommodation  — 
How  to  measure  relative  accommodation — How  to  plot  relative 
accommodation — Other  methods  of  measuring  relative  accommoda- 
tion— Relative  accommodation  influenced  by  age — How  is  relative 
accommodation  measured  for  clinical  purposes? — Clinical  import- 
ance of  relative  accommodation. 

DIVISION  IV. 

RELATIVE  CONVERGENCE ,  341 

Definition  of  relative  convergence — Desiderata  for  the  accurate 
measurement  of  relative  convergence — How  to  measure  relative 
convergence — How  to  record  relative  convergence — How  is  relative 
convergence  measured  for  clinical  purposes  ? — Its  clinical  import- 
ance. 


Contents  xi 


DIVISION  V 

TRUE  TORSION  WITH  CONVERGENCE 348 

Appliances  for  measuring  torsion  with  convergence — Le  Conte's 
— Author's — How  great  is  torsion  with  convergence  in  the  hori- 
zontal plane  ? — How  great  is  torsion  above  or  below  the  horizontal 
plane  ? — How  to  plot  torsion — Relative  torsion — Desiderata  for 
testing  relative  torsion — Imperfection  of  our  data — Object  of 
torsion  with  convergence — What  if  normal  torsion  is  artificially 
disturbed  ? — Clinical  value  of  such  measurements. 

CHAPTER  IX. 

BALANCE  OF  THE  OCULAR   MUSCLES 366 

Factors  in  the  production  of  comfortable  vision  at  the  near  point — 
Diagrammatic  representation  of  muscle  balance — Muscle  balance. 

CHAPTER  X. 

RELATION    OF   THE    "  GENERAL  STRENGTH  "    TO   THE    OCULAR    MUSCLES,         372 

CHAPTER  XI. 

RECAPITULATION  AND   CONCLUSIONS 376 

APPENDIX  A. 

BIBLIOGRAPHY 387 

APPENDIX  B. 

QUESTIONS   FOR    STUDY  .  ...  .  .  ,  .  .  .         435 

APPENDIX  C. 

OPHTHALMOLOGICAL  PERIODICALS   IN   SOME    AMERICAN  LIBRARIES  .         439 

INDEX   OF   AUTHORS 445 

INDEX    OF    SUBJECTS        ..........         448 

BIOGRAPHIC  NOTES 457 


I. 

ANATOMY  AND  PHYSIOLOGY. 


PART  I. 
ANATOMY. 

CHAPTER  I. 
THE  EXTRAOCULAR  MUSCLES. 

§  i.  Reason  for  Reviewing  the  Anatomy  of  the 
Muscles. — An  acquaintance  of  more  than  thirty  years  with 
ophthalmologists  in  different  countries  has  convinced  me 
that  the  study  of  the  anatomy  of  the  ocular  muscles  is  usu- 
ally sadly  neglected.  Although  an  ophthalmic  surgeon  may 
have  made  tenotomies  several  hundred  times,  his  further 
knowledge  of  the  muscles  is  too  often  acquired  from  the  in- 
spection of  a  few  dissections,  or  from  illustrations  in  standard 
text-books.  This  general  neglect  of  the  anatomy  is  cer- 
tainly a  cause,  and  with  the  corresponding  neglect  of  physi- 
ology is  probably  the  most  important  cause  of  our  present 
ignorance  and  confusion  clinically  concerning  this  subject. 
Any  one  who  attempts  to  make  dissections  of  the  ocular 
muscles  will  find  suggestions  as  to  modern  appliances  and 
methods  very  helpful,  but  on  searching  he  will  also  realize 
the  paucity  of  literature  on  the  subject.  In  view  of  the  im- 
provements in  technique  which  have  come  into  vogue  more 
recently  it  is  worth  while  to  refer  to  details  here  which  at 
first  may  appear  suited  only  to  a  beginner. 


2  Instruments  for  Dissection 

§  2.  Instruments  for  Dissection,  Preserving  Fluids, 
etc. — In  order  to  make  satisfactory  dissections  of  the  orbit 
it  is  necessary  to  have : 

1.  A  back  saw,  for  separating  the  skullcap. 

2.  A  chisel  or  chisel-hook. 

3.  A  fine  scroll  or  so-called  "jig  "  saw. 

4.  Stout,  straight  bone  forceps. 

5.  Curved  bone  forceps  or  ordinary  wire  nippers. 


FIG.  i.  —  Instruments  for  dissection  of  the  orbit  and  muscles. 

6.  A  circular  saw  two  or  three  inches  in  diameter. 

7.  A  dentist's  drill. 

8.  Half  a  dozen  small  blunt  probes. 

9.  A  thin  spatula. 

10.  Three  pairs  of  dissecting  forceps,  large,  medium,  and 
very  fine. 

11.  Half  a  dozen  scalpels,  large,  medium,  and  small. 

12.  One  elevator  or  gouge. 

13.  Two  bistouries. 

14.  Three  pairs  of  scissors,  heavy,  medium,  and  small — 
the  last  two  being  finely  pointed. 


Preserving  Fluids  3 

15.  A  dozen  or  more  steel  pins  of  assorted  sizes. 

16.  Half  a  dozen  small  self-closing  forceps. 

17.  A   piece   of   cork,  about    one  centimeter   thick   and 
fifteen  or  twenty  centimeters  square,  upon  which  the  speci- 
men can  be  fixed  during  dissection. 

1 8.  A  small  iron  vise. 

19.  An  Anel's  lacrymal  syringe. 

The  more  important  of  these  instruments  are  seen  in  Fig.  i. 

Preserving  Fluids — It  is  impossible  to  complete  a  good  dis- 
section of  theorbit  before  decomposition  assails  the  specimen. 
In  winter  this  process  can  be  delayed  by  proper  precautions, 
but  preserving  fluids  are  always  convenient  and  sometimes 
essential.  Quite  a  number  of  formulas  have  been  proposed 
for  this  purpose,  but  none  is  entirely  satisfactory. 

A  strong  solution  of  salt  water,  or  a  lo-per-cent.  solution 
of  carbolic  acid  in  glycerine,  will  keep  the  specimen,  though 
the  color  changes  and  dissection  is  rather  difficult.  The 
preservative  which  was  most  used  formerly  was  a  lo-per- 
cent.  solution  of  chloral  hydrate  in  water;  but  few,  if  any, 
changes  of  this  solution  are  necessary  for  preparations  of 
the  orbit,  but  the  jar  must  be  large  enough  to  hold  a  gen- 
erous supply.  More  recently,  a  4-per-cent.  solution  of 
formalin  came  into  vogue  and  has  proved  one  'of  the 
simplest  and  the  best.  It  has  the  disadvantage,  however, 
like  most  others,  of  rendering  the  parts  hard,  and  further 
dissection  somewhat  difficult. 

Kaiserling's  method  also  makes  the  specimen  hard,  but 
not  to  so  great  an  extent  as  most  of  the  other  mixtures.1 
It  has  the  great  advantage,  however,  when  properly  em- 
ployed, of  preserving  quite  well  the  color  of  the  parts  and 
for  that  reason  deserves  special  attention. 

There  are  three  steps  in  the  process. 

First.  The  specimen  is  arranged  in  the  desired  position 
and  covered  with  a  solution  composed  of 

Formalin 200  c.c. 

Water 1000  c.c. 

Potassium  nitrate 15  grams. 

Potassium  acetate 30 

1  Virchow's  Archiws,  1897,  p.  396. 


4  Injection  Fluids 

Kaiserling  says  that  the  specimen  should  be  left  in  this 
from  four  to  six  days,  but  I  have  found  that  two  or  three 
days  are  quite  sufficient  for  preparations  of  the  orbital 
muscles.  During  this  stage  it  is  particularly  desirable  to 
keep  the  specimen  in  the  dark.  Indeed,  the  color  of  the 
muscles  tends  to  fade  at  any  time,  unless  care  be  taken  to 
protect  them  from  continued  exposure  to  bright  light. 

Second.  The  specimen  is  placed  in  alcohol  to  bring  back 
the  color  of  the  blood.  It  has  been  found  that  if  a  dissec- 
tion be  allowed  to  remain  for  an  hour  or  so  in  8o-per-cent. 
alcohol  and  then  for  another  hour  or  more  in  95-per-cent. 
the  best  results  are  obtained. 

Third.  The  specimen  is  then  placed  in 

Water 2000  c.c. 

Potassium  acetate 200  grams. 

Glycerine 400  c.c. 

Here  it  remains  permanently,  care  being  taken  to  protect  it 
from  bright  light. 

A  considerable  saving  can  be  made,  if  desired,  in  the 
quantity  of  chemicals  used  in  the  solution.  If  this  be 
filtered  and  freshened  with  about  a  fourth  of  its  bulk  of  new 
fluid,  the  same  solution  can  be  used  a  number  of  times. 
Taken  all  together,  this  method  of  Kaiserling's  is  not  only 
the  best  thus  far  proposed  for  preserving  preparations  of  the 
ocular  muscles  when  it  is  desirable  to  retain  the  color  of 
the  parts,  but  it  is  also  the  one  best  adapted  for  the  globe, 
either  in  a  normal  or  abnormal  condition. 

Injection  Fluids. — We  shall  see  later  that  to  make  a  good 
dissection  of  the  orbit  it  is  necessary  to  distend  the  globe 
with  air  or  with  some  mixture  which  hardens  promptly. 
One  of  the  best  preparations  for  the  latter  purpose  is 

Gelatine 25  grams. 

Water 100  c.c. 

This  can  be  injected  through  the  optic  nerve  by  means  of  an 
Anel's  lacrymal  syringe. 

The  space  between  the  optic  nerve  and  the  capsule,  and 


Hardening  or  Fixing  Fluids  5 

also  between  the  globe  and  the  capsule,  can  be  demonstrated 
by  filling  it  with  some  solution  similar  to  that  used  for  in- 
jecting arteries.  A  number  of  formulas  are  available  for  the 
purpose.  One  of  the  simplest  is  as  follows  (from  Motais): 

Purified  suet .' 100  grams. 

oil 10      " 

Turpentine 10      " 

This  is  colored  with  vermilion  or  Prussian  blue,  or,  better, 
made  with  India  ink  that  has  already  been  ground  with 
turpentine. 

Hardening  or  Fixing  Fluids. — These  are  required: 

(A)  In  order  to  preserve  more  perfectly  the  form  of  the 
globe  and  the  relation  of  the  parts  when  the  specimen  is  to 
be  decalcified ;  and 

(B)  As  the  first  step  in  the  study  of  connective  tissue 
fibers  (check  ligaments)  according  to  methods  proposed  by 
Van  Giesen,  Mallory,  and  by  those  which  I  have  used  (B  21). 

The  best  solution  for  this  purpose  is  the  one  known  as 
Zcnker's  Fluid,  which  is  familiar  to  all  who  are  accustomed 
to  histological  technique.  This  is  composed  of 

Potassium  bichromate 2.5  grams. 

Sodium   sulphate i       gram. 

Mercuric  chloride 5       grams. 

Glacial  acetic  acid 5  c.c. 

Water  to 100  c.c. 

Sublimate  Solution. — Another  hardening  fluid  which  is 
useful  for  several  methods  of  staining  consists  of  a  saturated 
solution  of  corrosive  sublimate.  It  is  well  also  to  add  to 
this  .o5-per-cent.  of  glacial  acetic  acid.  Sublimate  requires 
a  longer  time  and  does  not  penetrate  the  specimen  so  thor- 
oughly, but  it  is  free  from  the  deep  yellow  stain  of  the 
Zenker  fluid. 

Alcohol. — A  95-per-cent.  solution  of  alcohol  is  still  occa- 
sionally used  for  preserving  and  fixing  the  specimen,  if  the 
student  desires  to  study  the  check  ligaments  and  the  con- 
nective tissue  by  means  of  the  stains  proposed  by  Ribbert  or 
by  Unna.  This,  however,  does  not  give  the  best  results, 


6  Decalcification 

and  alcohol  is  now  largely  superseded  in  the  laboratory  by 
other  preserving  fluids. 

Formalin. — The  specimen  may  be  hardened  in  a  4-per- 
cent, solution  of  formalin  or  in  the  manner  proposed  by 
Kaiserling,  but  this  is  not  adapted  to  the  subsequent  stain- 
ing of  the  connective  tissue. 

Decalcification. — It  is  often  necessary  in  making  microscopi- 
cal sections  to  have  the  bony  parts  of  the  orbit  thoroughly 
decalcified.  The  process  is  an  important  one  and  if  im- 
properly done  the  specimen  may  be  injured  or  ruined 
entirely.  As  a  preliminary  step  it  is  desirable  to  harden 
the  part  in  one  of  the  fixing  solutions  just  described.  As 
to  the  various  acid  mixtures  used  for  decalcifying,  a  number 
of  trials  indicate  that  the  directions  given  in  most  of  the 
books  on  histology  are  misleading,  when  applied  to  studies 
of  the  orbit  and  to  the  eye.  The  difficulty  is  that  these 
mixtures  are  too  strong. 

If  the  bone  around  the  opening  of  the  orbit  be  cut  down 
to  a  thin  layer  in  the  first  place,  and  if  an  abundant  supply 
of  the  mixture  be  used,  or  if  it  be  changed  every  three  or 
four  days,  the.  specimen  will  be  ready  for  further  dissection 
at  the  end  of  two  or  three  weeks.  As  soon  as  decalcifica- 
tion  is  complete  the  specimen  should  be  removed  from  the 
acid  (as  disintegration  continues  afterward),  and  it  can  then 
be  kept  in  6o-per-cent.  alcohol  for  further  study.  A  certain 
amount  of  softening  occurs  almost  inevitably.  It  can,  how- 
ever, be  reduced  to  the  minimum  by  enveloping  the  bony 
pyramid  in  absorbent  cotton  or  cloth,  and  then  dropping 
upon  this  enough  of  a  2o-per-cent.  or  even  a  3O-per-cent. 
solution  of  hydrochloric  acid  to  keep  the  cotton  saturated. 
With  care  in  doing  this  the  decalcification  can  be  completed 
rapidly  and  with  but  little  effect  on  the  soft  parts. 

The  formula  given  by  Pereyni  is  excellent.  It  is  as 
follows : 

4  parts  of  a  lo-per-cent.  solution  of  nitric  acid. 

3  parts  of  a  95-per-cent.  alcohol. 

3  parts  of  a  o.5-per-cent.  solution  of  chromic  acid. 

As  it  contains  no  corrosive  sublimate,  however,  it  is  use- 
less for  those  stains  which  involve  the  mercury  reaction. 


The  Extraocular  Muscles  7 

§  3.  Dissections  of  the  Orbits  of  Animals. — These  offer 
an  excellent  opportunity  for  practising  methods  of  dissec- 
tion. It  might  be  taken  for  granted  that  doctors  of  medi- 
cine can  make,  without  difficulty,  any  dissection  of  the 
orbit  which  may  be  desired,  but  unfortunately  a  few  at- 
tempts usually  prove  that  such  is  not  the  case.  There  are 
details  of  technique  to  be  followed  which  are  not  always 
learned  in  the  college  dissecting-room,  or,  even  if  they  have 
been,  are  probably  forgotten.  It  is,  therefore,  much  easier 
for  one  to  learn  these  methods  on  the  eyes  of  animals  before 
attempting  exact  dissections  of  human  orbits.  Again,  such 
dissections  give  an  opportunity  to  become  familiar  with 
preserving  fluids  and  with  various  other  details  which  can 
be  acquired  only  by  laboratory  experience. 

Finally,  the  results  are  interesting  in  themselves,  showing 
how  the  same  general  plan  of  arrangement  of  the  muscles  is 
followed  in  the  orbits  of  most  of  the  vertebrates. 

§  4.  Dissection  of  the  Human  Orbit. — Having  selected 
a  subject  which  has  but  little  adipose  tissue,  the  orbits  or 
their  contents  may  be  removed  by  two  or  three  different 
plans.  One  of  the  simplest  is 

(A)  The  removal  of  the  orbits  with  their  contents.  The 
details  of  this  are  as  follows : 

First.  Take  off  the  calvaria.  In  doing  so,  make  the  in- 
cision across  the  frontal  bone,  passing  within  a  centimeter 
of  the  supraorbital  ridge. 

Second.  Make  an  incision  parallel  to  the  first  one  and 
below  it,  beginning  near  the  middle  of  the  nose,  passing 
backward  just  beneath  the  lower  margin  of  the  orbit  and 
extending  almost  to  the  ear. 

Third.  Another  incision  with  the  saw  extends  from  the 
extremity  of  the  last  one  and  is  perpendicular  to  it.  This 
detaches  the  part  which  includes  the  orbits  with  their  con- 
tents. 

Fourth.  The  orbits  are  separated  from  each  other  by  a 
section  in  the  median  line,  and  each  orbit  is  then  cleared  of 
superfluous  bone.  In  doing  this  it  is  advisable  to  hold  the 
specimen  firmly  in  a  small  vise  and  trim  off  fragments  of  the 
ethmoid,  or  parts  of  the  edges  of  the  orbit  which  are  thick, 


8  Dissection  of  the  Orbit 

by  means  of  the  nippers  and  the  scroll-saw,  leaving  only  the 
pyramid  with  its  bony  covering. 

Fifth.  The  next  step  is  to  open  this  pyramid.  If  it  be 
desired  to  study  first  the  origin  of  the  long  muscles,  the 
pyramid  can  be  truncated  with  the  saw  fifteen  or  twenty 
millimeters  from  the  apex.  But  usually  it  is  better  to  re- 
move first  the  roof  of  the  orbit,  then  the  outer  or  inner  wall, 
and  lastly,  to  detach  the  apex  with  the  muscles  which  arise 
around  the  optic  foramen.  With  this  in  view,  it  is  well  to 
cut  first  a  small  window  in  the  roof  with  the  saw.  The 
periosteum  here  is  very  loosely  attached  and  it  is  only 
necessary  to  pass  a  pair  of  strong  forceps  between  it  and 
the  bone,  allowing  the  blades  to  open.  In  this  way  the 
periosteum  can  be  detached  as  far  forward  as  the  frontal 
ridge.  After  that,  the  roof  can  be  broken  off  piece  by  piece 
with  the  nippers,  care  being  taken,  however,  not  to  inter- 
fere with  the  pulley  of  the  superior  oblique. 

Sixth.  The  specimen  can  now  be  fastened  with  pins  to 
a  table,  or,  still  better,  to  a  piece  of  cork  on  the  table, 
and  the  student  can  proceed  with  the  dissection  of  the 
muscles  or  other  parts  of  the  orbit. 

(B)  Removal  of  the  orbital  contents  with  the  lids  and 
ligaments  and  with  a  ring  of  bone.  This  method  is  easier 
to  describe  than  to  execute.  An  incision  is  first  made 
with  a  scalpel  through  the  soft  parts  to  the  bone,  about  a 
centimeter  from  the  edge  of  the  orbit  and  entirely  around 
it.  This  incision  is  then  deepened  by  means  of  the  circular 
saw.  Above,  it  passes  first  into  the  frontal  sinus,  and  then, 
going  deeper,  it  reaches  the  periosteum.  Care  should  be 
exercised,  at  this  point,  not  to  injure  the  levator  palpebrae 
or  deeper  portions,  and  it  is  therefore  necessary  to  feel 
one's  way  cautiously  with  a  small  blunt  probe. 

Having  reached  the  periosteum  the  incision  made  by  the 
circular  saw  is  extended  toward  the  external  canthus,  fol- 
lowing the  curve  of  the  orbital  ridge.  After  the  cut  has 
advanced  even  a  short  distance,  a  blunt  probe  can  be  readily 
introduced  into  the  opening,  separating  the  periosteum  from 
the  bone.  Sometimes  it  is  desirable  to  pass  through  the 
opening  a  flattened  probe,  or  even  a  thin  spatula,  in  order 


The  Extraocular  Muscles  9 

to  separate  the  periosteum  from  the  bone.  If  neither  of 
these  can  be  passed  through  the  first  incision,  a  second  one 
is  made  parallel  to  the  first  and  about  half  a  centimeter 
above.  In  this  way  the  incision  is  extended  from  one  end 
of  the  orbital  ridge  to  the  other.  At  its  inner  extremity, 
in  order  to  avoid  injury  to  the  pulley  of  the  superior  oblique, 
the  cut  must  be  almost  horizontal.  At  the  outer  and  inner 
angles  it  is  necessary  to  make  rather  an  abrupt  turn,  the 
incision  passing  almost  perpendicularly  to  the  one  on  the 
supraorbital  ridge.  At  this  point  the  circular  saw  is  rather 
unsatisfactory,  and  it  is  better  to  use  a  sliarp  chisel. 

The  incision  along  the  lower  margin  of  the  orbit  can  be 
made  in  the  same  way  as  that  along  the  upper  margin.  But 
even  after  this  is  accomplished,  care  and  patience  are  still 
necessary  in  order  to  complete  the  dissection  of  the  peri- 
osteum from  the  bone,  especially  in  the  lower  and  outer 
portion,  along  the  line  of  the  sphenoidal  sinus.  Finally, 
when  the  rim  of  bone  is  loosened,  a  pair  of  thin  scissors  can 
be  passed  into  the  orbit,  then  cutting  the  optic  nerve  with 
the  muscles,  the  orbital  tissues  come  away  all  together. 
This  method  is  to  be  selected  only  when  permission  can  not 
be  obtained  to  open  the  skull,  but  when,  in  spite  of  that,  it 
is  desired  to  study  quite  exactly  the  check  ligaments  and 
the  insertions  of  the  muscles. 

(C)  Removal  of  the  orbital  contents  with  the  lids  and 
ligaments,    but   without    the   edge   of  the   orbit.     This  is 
always  easy,  although  the  specimen  is,  of  course,  very  in- 
complete.    The  incision  begins  at  the  root  of  the  nose,  over 
the  lacrymal  sac,  and,  arching  upwards,  follows  the  orbital 
ridge  just  below  the  line  of  the  brow,  then,  curving  down- 
wards, it  passes  inward  along  the  lower  margin  of  the  orbit 
to  the  place  of  beginning.     This  incision  is  deepened  to  the 
bone.     The  knife  is  laid  aside  and  the  periosteum  lifted  off 
carefully  with  the  elevator.     It  is  slow  work  over  the  edges 
of  the  orbit,  but  when  once  past  that  the  attachments  to 
the  bone  are  slight,  except  near  the  deeper  portions.     The 
mass  can  then  be  separated  by  a  few  cuts  with  the  scissors 
and  removed. 

(D)  Removal  of  the  muscles  without  their  ligaments.     It 


io  Inflation  of  the  Globe 

happens  frequently  that  our  dissection  must  be  limited  to 
incisions  which  leave  as  little  trace  as  possible.  Quite  a 
satisfactory  specimen  can  be  obtained  by  making  an  incision 
ten  or  fifteen  millimeters  long  straight  out  from  the  ex- 
ternal canthus;  then,  at  the  extremity  of  this  incision, 
another,  at  right  angles  to  the  first  and  about  the  same 
length.  Both  together,  therefore,  form  a  letter  T  lying  on 
its  side.  The  next  step  is  to  evert  the  lids  and  dissect  out 
the  contents  of  the  orbits.  A  little  patience  is  necessary 
in  doing  this,  especially  where  the  tissue  is  dense  near  the 
outer  and  inner  angles  of  the  eye;  the  cuts  should  be  made 
carefully,  and  it  is  better  to  snip  one's  way  along  with 
pointed  scissors  than  to  advance  with  a  scalpel.  When  this 
has  been  accomplished,  the  contents  of  the  orbit  can  be  re- 
moved as  before  described. 

When  we  are  not  permitted  to  disfigure  the  face  of  the 
subject  in  any  way,  it  is  still  possible  to  remove  the  greater 
part  of  all  of  the  muscles  by  following  this  same  plan,  with- 
out first  cutting  the  canthus.  The  specimen,  however,  is 
not  very  satisfactory  and  lacks  so  much  as  hardly  to  repay 
the  trouble  of  exact  preparation  and  dissection. 

§  5.  Inflation  of  the  Globe.— Immediately  after  death 
the  cornea  becomes  flaccid,  the  globe  loses  its  tonicity,  and 
as  the  intraocular  fluids  evaporate  it  gradually  sinks  into 
the  orbit.  In  all  specimens  which  are  not  perfectly  fresh 
the  globe  has  ceased  to  retain  its  normal  relation  to  the 
muscles  and  other  surrounding  tissues.  Especially  is  this 
a  characteristic  of  cadavers  which  have  been  frozen,  and  as 
every  dissecting-room  now  has  a  freezing  apparatus,  human 
eyes  which  are  available  for  study  are  often  thus  sunken 
and  contracted.  It  therefore  becomes  necessary  to  restore 
the  globe  to  its  normal  form  and  thus  reestablish  the  rela- 
tions of  the  muscles  attached  to  it. 

The  most  effective  method  of  doing  this  is  to  inflate  it. 
The  process  is  simple.  The  orbital  contents  having  been  re- 
moved by  following  any  one  of  the  plans  already  detailed, 
the  student  turns  the  apex  of  the  cone  towards  him,  and 
with  two  pairs  of  forceps  carefully  picks  out  the  optic  nerve. 
This  nerve,  with  its  sheath  and  with  the  connective  tissue 


The  Extraocular  Muscles  n 

surrounding  it,  is  dissected  out  for  a  half  or  a  third  of  its 
length.  A  long,  straight,  triangular  needle  is  passed  into 
the  center  of  the  nerve,  and  by  twisting  the  needle  on  its 
axis  the  nerve  tissue  oozes  out  from  its  sheath.  On  reach- 
ing the  point  where  the  optic  nerve  passes  into  the  globe 
considerable  resistance  is  offered  by  the  lamina  cribrosa, 
but  after  passing  that  the  globe  is  easily  punctured.  The 
needle  is  then  withdrawn,  a  blow-pipe  is  inserted  through 
the  canal  thus  made,  and  a  stout  linen  thread  having  been 
passed'around  the  nerve  sheath,  the  globe  is  inflated  and  at 
the  same  moment  the  knot  of  the  thread  is  tied.  With  the 
return  of  the  globe  to  its  normal  form,  the  muscles  also 
resume  their  normal  relations. 

§  6.  Lines  of  Origin  at  the  Apex  of  the  Orbit.1 — In  any 
study  of  the  ocular  muscles  it  is  natural  to  begin  with  their 
origins  and  to  examine  the  point  from  which  most  of  them 
spring.  For  this  purpose  it  is  convenient  to  truncate  the 
cone  of  an  orbit  which  has  been  properly  preserved  in  the 
Kaiserling  or  some  similar  fluid,  when  with  a  little  dissec- 
tion we  can  easily  see  the  relation  of  the  parts  to  each  other. 
When  coming  now  to  the  names  of  the  muscles  it  should  be 
observed  that  it  would  accord  better  with  modern  nomen- 
clature to  describe  the  rectus  medialis  instead  of  the  rectus 
internus.  But  English-speaking  students  know  that  muscle 
as  the  internal — not  as  the  median  rectus,  and  that  fact  must 
be  accepted  until  changed  by  some  formal  agreement  among 
ophthalmologists  as  well  as  anatomists. 

Of  the  six  muscles  which  move  the  eye,  five  of  these,  and 
also  the  levator  palpebrae,  arise  from  the  apex  of  the  orbit. 
Some  confusion  exists  as  to  the  relative  position  of  these 
origins,  caused  to  a  great  extent,  as  Dwight  (B  20)  has  ob- 
served, by  the  unnecessary  complications  in  different  de- 
scriptions. These  complications  are  of  comparatively  recent 

1  In  most  of  the  works  on  descriptive  anatomy  it  is  customary  to  follow 
each  muscle  in  turn  from  origin  to  insertion.  For  our  purpose,  however,  it  is 
simpler,  and  from  the  clinical  standpoint  it  is  better,  to  consider  first  the  com- 
mon origin  of  those  which  come  from  the  apex  of  the  orbit,  then  each  muscle 
in  detail,  and,  finally,  to  study  the  primary  and  secondary  insertions  of  each,  and 
the  relations  of  all  to  the  fascia  or  ligaments  which  connect  them  to  the  adja- 
cent tissue  or  to  the  margin  of  the  orbit. 


I  2 


Lines  of  Origin 


o, 


date,  for  one  of  the  earliest  anatomists,  Zinn,  describes  the 
muscles  as  arising  from  what  he  called  the  annulus  tendinem 
communis.  This  tendinous  "ring,"  so  called,  surrounds  the 
optic  nerve,  and  from  it  the  greater  number  of  the  muscles 
spring,  each  one  from  the  part  of  the  ring  corresponding  to 
the  portion  of  the  eye  into  which  the  muscle  is  inserted. 
Fig.  2  shows  the  lines  of  origin  of  these  muscles. 

It  will  be  seen  that  there  is  not  in 
reality  a  simple  ring  around  the  optic 
nerve.  The  lines  of  the  muscular  ori- 
gins, when  taken  altogether,  may  be 
compared  rather  to  the  figure  eight 
tipped  toward  the  median  line.  The 
inner  and  upper  loop  of  this  figure, 
being  somewhat  the  larger,  surrounds 
the  optic  nerve;  the  lower  and  outer 
loop,  the  one  around  the  foramen,  is 
that  through  which  the  motor  oculi 
nerve  makes  its  entrance  into  the  or- 
bit. (Fig.  2.) 

The  origin  of  the  superior  rectus 
forms  the  greater  part  of  the  upper 
side  of  this  figure  eight,  some  of  the 
fibers  running  down  between  the  two 
openings  for  the  optic  and  for  the  mo- 
tor oculi  nerves.  The  median  portions 
of  this  loop  or  circle  around  the  optic 
foramen  are  formed  by  some  of  the 
fibers  of  the  internal  rectus.  The  ori- 
gin of  the  inferior  rectus  is  situated  below  these  two  loops, 
near  their  junction. 

The  external  portion  of  the  outer  loop  is  formed  by  one 
head  of  the  externus.  This  muscle,  however,  has  another 
head,  which  arises  from  the  outer  margin  of  the  superior 
orbital  plate  of  the  sphenoid.  In  this  way  the  muscles 
surround  the  foramina  for  the  entrance  of  the  optic  and  the 
motor  oculi  nerves  into  the  orbit.  The  levator  palpebrae 
arises  from  a  curved  line  situated  nearer  the  upper  and  inner 
margin  of  the  orbit,  the  line  of  origin  being  almost  con- 


Rm. 
Mi. 

FlG.  2. — Lines  of  ori- 
gin of  the  muscles  at  the 
apex  of  the  orbits  (Mer- 
kel).  In  this  figure  and 
in  others  which  follow, 
taken  from  continental 
anatomists,  J?  means  rec- 
tus, m,  medialis  or  in- 
terims, /,  lateralis  or 
externus.  Other  letter- 
ing usually  the  same  as 
in  English  and  American 
text-books. 


The  Extraocular  Muscles 


centric  with  the  optic  foramen.     The  arrangement  of  the 
muscles  themselves  at  the  apex  of  the  orbit  is  seen  in  Fig.  3. 

§7.  Meaning  of  the  "Prim-  AJ.  Lf 

ary  "  and  "  Secondary  "  Inser- 
tions, and  Other  Preliminary 
Considerations.— Having  thus 
glanced  at  the  origins  of  the 
extraocular  muscles  which  arise 
at  the  apex  of  the  orbit,  we  are 
ready  to  consider  each  one  in 
turn.  But  when  doing  so,  as 
we  pass  to  the  attachment  of 
each  into  the  globe,  mention 
must  be  made  of  the  "Prim- 
ary" and  "Secondary"  inser- 
tions. As  those  terms  will  be 
new  to  most  readers,  it  is  de- 
sirable here  to  give  an  idea  of 
what  is  meant,  although  they 
will  be  considered  more  in  de- 
tail later. 

It  happens  frequently  throughout  the  body  that  the  in- 
sertion of  a  muscle  is  strengthened  by  lateral  extensions 
more  or  less  complete,  or  by  two  or  more  distinct  points  of 
attachment.  Ordinarily  these  supplementary  insertions  are 
of  no  special  importance  physiologically  or  clinically.  With 
the  ocular  muscles,  however,  we  shall  see  that  even  small 
fibers  of  connective  tissue,  so  small  as  to  be  seen  with  diffi- 
culty, and  not  situated  directly  in  the  line  of  insertion, 
may  affect  the  position  of  the  globe  to  an  important  de- 
gree. With  our  increasing  knowledge  of  these  accessory 
fibers  it  becomes  clear  that  we  should  study  them  as  carefully 
as  we  study  those  other  fibers,  which,  being  joined  together, 
form  the  tendon  going  to  the  main  insertion.  We  must 
therefore  take  into  account  not  simply  the  primary  or 
principal  insertion,  but  also  these  accessory  insertions  of  the 
muscles  which  may  properly  be  called  secondary. 

It  may  seem  an  unnecessary  refinement  to  give  as  much 
attention  to  details  as  will  be  found  presently  in  the 


FIG.  3. — Apex  of  the  orbit  with 
muscles  in  position.  Ks,  rectus 
superior  ;  Lp,  levator  palpebrae  ; 
Os,  obliquus  superior ;  fil,  rectus 
externus ;  J?i,  rectus  inferior ; 
Rm^  rectus  internus  ;  No,  nervus 
opticus  (Merkel  and  Kallius). 


14  Primary  and  Secondary  Insertions 

measurement  of  these  secondary  insertions  or  in  the  exact 
size  of  the  different  muscles.  But  second  thought  shows  the 
value  of  such  care.  Thus,  it  has  occurred  to  every  operator, 
while  doing  a  tenotomy,  to  find  that  a  division  of  the  merest 
thread  of  connective  tissue  will  make  a  very  decided  differ- 
ence in  the  result,  and  if  we  know  what  to  expect  in  the 
average  case  as  to  the  position  and  length  of  the  line  of  in- 
sertion it  is  an  evident  assistance.  Or  if,  after  making 'the 
division  of  the  tendon,  the  surgeon  finds  that  the  line  of 
insertion  is  placed  obliquely  to  its  ordinary  position,  and 
that  he  has  to  do  with  an  extreme  variation,  he  infers  that 
other  muscles  must  also  be  inserted  in  unusual  positions  in 
order  to  produce  a  proper  compensating  effect.  In  other 
words,  such  a  case  will  probably  require  subsequently  more 
than  ordinary  care  either  by  some  of  the  non-operative 
methods  of  treatment,  or  possibly  by  further  operations. 
In  a  similar  manner  the  number  of  square  millimeters  ex- 
posed when  a  muscle  is  divided  transversely  should  also  be 
taken  into  account.  This  means,  of  course,  the  contracting 
power  of  one  as  compared  with  that  of  its  antagonist. 
When  we  come,  later,  to  consider  the  different  reasons 
which  have  been  given  by  Graefe,  by  Hansen  Grut,  and 
others  to  account  for  the  various  forms  of  apparent  devia- 
tions, it  will  be  seen  that  the  relative  strength  of  different 
muscles  or  groups  of  muscles  is  no  small  factor  in  deciding 
these  important  questions  of  etiology.  Or,  again,  as  to 
these  details  of  the  primary  or  secondary  insertions,  it  is 
probable  that  the  disregard  of  such  points  is  one  of  the 
main  reasons  of  our  confusion  and  ignorance  concerning 
certain  aspects  of  this  part  of  ophthalmology. 

When  considering  the  extraocular  muscles  it  is  necessary 
to  bear  in  mind  a  peculiarity  in  their  structure  to  which  at- 
tention has  been  called  recently  by  Schiefferdecker  (B  24). 
It  has  long  been  known  that  in  various  portions  of  the  body 
elastic  connective  tissue  fibers  are  found  both  in  the  epi- 
mysium  and  also  in  the  perimysium.  A  careful  examination 
by  this  observer,  however,  shows  that  such  elastic  fibers  are 
much  more  abundant  in  the  ocular  muscles  than  elsewhere, 
this  being  especially  noticeable  in  the  superior  rectus.  The 


The  Extraocular  Muscles  15 

general  direction  of  these  elastic  fibers  is  usually  the  same  as 
that  of  the  muscle,  but  they  also  intertwine  with  each  other, 
often  forming  a  network  of  anastomoses.  It  is  probable  that 
the  abundance  of  these  fibers  in  the  ocular  muscles  assists 
materially  in  the  rapid  and  frequent  motions  of  the  globe. 

§  8.  The  Levator  Palpebrae. — After  the  pyramidal  con- 
tents of  the  orbit  have  been  removed  by  following  one  of 


di 


FlG.  4. — View  of  the  upper  surface  of  the  orbit,  showing  the  levator  palpebrae 
with  the  insertions  which  pass  outward  and  inward  (Merkel  and  Kallius). 

the  plans  already  described,  if  we  dissect  off  the  periosteum 
which  lines  the  roof  of  the  orbit,  we  find  a  muscle  lying  just 
beneath,  which  passes  from  the  apex  of  the  pyramid  almost 
straight  forward.  This  is  the  levator  palpebrae. 

It  arises  from  a  small  head  two  or  three  millimeters  above 
the  optic  foramen,  just  beneath  the  periosteum  which  lines 
the  roof  of  the  orbit,  and  then,  passing  forward,  spreads 
out  into  a  broad  tendon.  But  all  of  the  fibers  do  not  pass 
directly  to  the  cartilage  of  the  lid.  Those  near  the  outer 
edge  branch  off  toward  the  lacrymal  gland,  forming  a  firm 


i6 


The  Levator  Palpebrae 


network  supporting  it  (Figs.  4  and  5),  and  those  near  the 
inner  edge  bend  around  almost  at  right  angles  toward 
the  pulley  of  the  superior  oblique.  In  reality,  therefore, 
the  levator  palpebrae  has  not  a  single  insertion  into  the  edge 
of  the  superior  cartilage,  but  in  a  certain  sense  it  may  be 
said  to  have  three  insertions,  a  central,  external,  and 
internal. 

Let    us  examine  more  exactly  this   central   portion   oi 


FIG.  5. — Levator  palpebrae,  inferior  surface. 

the  fibers  of  the  levator.  The  earlier  anatomies  and  some 
of  the  general  text-books  still  describe  the  tendon  of  the 
levator  as  passing  forward  to  be  inserted  into  the  upper 
border  of  the  cartilage.  More  careful  study,  however, 
showed  that  this  was  not  quite  true.  The  fact  is  that  as 
the  tendon  of  the  muscle  approaches  the  cartilage  it  divides 
into  at  least  two  layers.  The  lower  one  of  these  passes  di- 
rectly to  be  inserted,  as  already  described,  into  the  upper 


• 

The  Extraocular  Muscles  17 

edge  of  the  cartilage.  This  small  band  of  fibers,  having 
been  described  first  by  Miiller,  is  called  after  him.  The 
upper  layer  of  the  tendon  passes  over  the  upper  edge  of 
the  cartilage,  and  is  inserted  into  its  upper  and  anterior 
surface,  where  the  fibers  blend  with  the  fibers  of  the  orbicu- 
laris.  These  two  divisions  of  the  muscle  are  well  defined 
and  are  figured  by  Schwalbe  (B  13)  as  quite  distinct. 

Recently  another  description  of  the  insertion  has  been 
given  by  Wolff  (B  25).  He  has  shown  that  the  anterior 
or  upper  layer  of  muscular  fibers  continues  over  the  whole 
anterior  surface  of. the  cartilage,  extending  almost  or  quite 
down  to  its  lower  border. 

The  method  of  insertion  of  the  tendon  of  the  levator  is 
of  evident  importance  in  all  of  those  operations  for  ptosis 
which  involve  in  any  way  the  insertion  of  the  muscle, — such, 
for  example,  as  the  operations  of  Motais,  Hotz,  Wolff,  the 
success  of  each  one  of  these  procedures  depending  upon  a 
knowledge  of  the  anatomical  relation  of  the  cartilage  to  the 
muscle. 

The  action  of  the  levator  is  really  to  lift  the  lid.  But  as 
the  path  of  the  muscle  from  its  origin  to  the  lid  is  not 
directly  forward,  but  also  outward,  its  natural  tendency 
would  be  to  draw  the  lid  up  and  inward.  That,  however, 
is  counteracted  by  the  two  lateral  groups  of  fibers. 

§  9.  The  Internal  Rectus  (Rectus  Medialis). — As  the 
internal  is  one  of  the  most  important,  clinically,  of  the  recti, 
we  will  consider  it  first.  It  arises  from  the  inner  and  lower 
border  of  the  ligament  of  Zinn,  having  its  upper  edge  in 
contact  with  the  origin  of  the  levator  palpebrae  and  the 
superior  rectus.  Its  lower  margin  is  in  contact  at  first  with 
the  origin  of  the  inferior  rectus,  while  still  farther  to  the 
inner  side,  and  lying  close  to  the  wall  of  the  orbit,  is  the 
origin  of  the  superior  oblique.  The  internal  is  smaller  near 
its  origin  than  the  external  rectus,  but  soon  expands  into  a 
fleshy  belly.  As  it  passes  forward  it  is  in  close  contact 
with  the  internal  wall  of  the  orbit,  having  its  upper  edge  at 
first  almost  continuous  with  the  superior  oblique.  But  as 
the  muscles  advance,  the  internal  rectus  continues  horizon- 
tally forward  toward  its  insertion  into  the  sclerotic,  while 


i8 


The   Internal   Rectus 


the  superior  oblique  is  directed  toward  the  pulley  at  the 
upper  and  inner  margin  of  the  orbit.  The  relations  of  the 
muscles  in  the  orbit  can  be  seen  best  in  frozen  transverse 

sections.  These  are  very 
easily  made,  especially  in  win- 
ter, the  following  drawings  of 
such  sections  being  taken  from 


F.  N. 


P.N. 


Inf.  O.  N. 


FIG.  6. — Frontal  section  of  frozen 
right  orbit,  about  twelve  millimeters 
behind  globe.  Seen  from  behind. 
S.  R.,  superior  rectus  ;  L.  P.,  levator 
palpebrse  ;  S.  O.,  superior  oblique; 
In.  R.,  internal  rectus  ;  Inf.  R.,  in- 
ferior rectus  ;  E.  R.,  external  rectus  ; 
O.  N.,  optic  nerve;  F.  N.,  frontal 
nerve;  Inf.  O.  N.,  infraorbital  nerve; 
6th,  sixth  nerve. 


Inf.  O.  N. 


FIG.  7. — Section  about  five  millime- 
ters behind  globe.  Letters  as  in  pre- 
ceding figure. 


Dwight  (B  20).  The  relations  of  the  internal  rectus  vary 
somewhat.  Thus,  near  its  origin,  it  lies  rather  to  the  lower 
and  to  the  inner  side  of  the  orbit ;  above  it  and  to  the  inner 
side  is  the  superior  oblique,  and  above  and  externally  is  the 
optic  nerve,  from  which  it  is  separated  by  a  cushion  of  fat. 
Below  and  internally  is  the  wall  of  the  orbit,  with  which  it 
lies  in  contact.  When  the  muscle  has  advanced,  however, 
to  within  five  or  six  millimeters  behind  the  eye,  its  relation 
is  somewhat  altered,  having  assumed  a  relatively  higher 
position.  Farther  forward,  the  relations  of  the  internal 
rectus  are  modified  still  more.  Then  the  superior  oblique 
lies  almost  above  it,  the  center  of  the  muscle  being  quite 
on  a  line  with  the  center  of  the  globe.  See  Figs.  6,  7,  8,  9. 
The  principal  insertion  '  of  the  internal  rectus  is  in  a  line 
slightly  convex  to  the  margin  of  the  cornea  or  almost  par- 
allel to  a  line  tangent  to  it.  The  center  of  the  insertion 


1  Figures  10,  ir,  12,  13,  14,  15,  16,  27,  and  28  are  from  my  dissections  and  the 
photographs  are  purposely  left  untouched. 


The  Extraocular  Muscles 


8.  O.  N. 


is  on  the  average  five  or  five  and  a  half  millimeters  from 
the  cornea,  near  the  point  where  the  globe  is  inter- 
sected  by  the  horizontal  plane  passing  through  its  center. 
In  a  few  instan- 
ces the  center  of 
this  insertion  is 
one  or  two  milli- 
meters below 
that  point — very 
rarely  above  it. 
Also,  the  line  of 
insertion  may  be 
inclined  so  that 
its  upper  end  is 


lo.  ft. 


E.E. 


•Inf  E. 
Inf.  O.N. 


FlG.  8, — Section  about  three  millimeters  in  front  of 
entrance  of  nerve.  Letters  as  before.  S.  O.  N., 
supraorbital  nerve. 


S.O 


In.R. 


S.  E. 


E.B. 


nearer  to  the  cor- 
nea than  the 
lower,  or  the  reverse.  Such  variations  from  the  type  are 
less  frequent  with  the  recti  than  with  the  oblique.  The 
secondary  insertions  of  the  internal  rectus  are  always  pres- 
ent and  naturally  are  of  importance  clinically.  From  each 
edge  of  the  tendon,  above  and  below,  fibers  of  connective 

tissue   spread    off 
toward  the   scle- 
rotic, so  that  some- 
times    when     the 
muscle    is    drawn 
out  at  right  angles 
to  the  globe  it  is 
difficult    to  deter- 
mine    at     exactly 
what    point    the 
tendon    proper  or 
Letters   tjie  primary  inser- 
tion ends,  and  just 
where  the   lateral 
connective  tissue  fibers,  or  secondary  insertions,  begin  (Fig. 
u).     On  certain  globes,  however,  these  secondary  exten- 
sions above  and  below  are  exceedingly  small  and  thin. 
Secondary  attachments  connecting  the  ocular  surface  of 


Inf.  B. 


FIG.  g. — Section  near  equator  of  globe, 
as  before.     Inf.  O.,  inferior    oblique;  L.  S.,    lacry- 
mal  sac. 


2O  The  Internal  Rectus 

the  internal  rectus  to  the  globe  are  always  present.  These 
fibers,  usually  very  minute,  are  situated  near  the  primary 
insertion,  passing  from  the  sheath  of  the  muscles  to  the 
outer  part  of  the  capsule  of  Tenon,  and,  it  is  needless  to 
say,  play  a  very  important  part  in  every  operation  of 
tenotomy  of  this  muscle. 

Secondary  attachments  on  the  median  or  orbital  side  of  the 
internal  rectus  are  also  important.  They,  too,  are  always 
present,  though  their  firmness  and  abundance  vary  in  dif- 


FIG.  10. — Internal  rectus,  median  surface;  the  superior  oblique  lies  just  to  its 
left.     The  superior  rectus  is  in  profile  to  the  extreme  left. 

ferent  subjects.  By  staining  horizontal  sections  properly, 
these  fibers  from  the  sheath  of  the  muscle  can  be  easily, 
seen  beginning  several  millimeters  posterior  to  the  principal 
insertion.  The  more  posterior  fibers  pass  almost  directly 
forward,  those  more  anteriorly  pass  both  forward  and 
inward,  and,  interlacing  with  other  fibers,  blend  with  the 
central  portion  of  the  fascia  orbito-ocularis.  In  this  posi- 
tion they  form  a  portion  of  what  is  called  the  internal  check 
ligament.  According  to  Schneller  the  rectus  internus  has 
the  following  dimensions :  length,  40.7  mm.;  width,  10.3  mm.; 


FIG.  it. — Internal  rectus,   ocular  surface, 
with  the  ocular  secondary  insertions. 


FIG.   12. — Internal  rectus  ;  superior  lateral   edge  is  held  in   the 
forceps. 


22  The  External  Rectus. 

thickness,  1.6  mm.     In  section  it  measures  17.4  square  mm. 
Its  volume  is  709.  cbmm.    Its  weight  is  0.747  grams.    (624.) 

§  10.  The  External  Rectus  (Rectus  Lateralis). — Un- 
like the  other  ocular  muscles  this  has,  as  already  mentioned, 
two  points  of  origin,  one  from  the  outer  margin  of  the  optic 
foramen,  and  the  other  from  the  ligament  of  Zinn  and  its 
extension.  A  small  opening  is  left  between  the  two  heads, 
and  through  this  pass  the  third  and  sixth  nerves,  and  the 
nasal  branch  of  the  fifth  together  with  the  ophthalmic 
vein.  The  two  heads  of  this  muscle  join  almost  immedi- 
ately, and  in  the  posterior  portion  of  the  orbit  form  the 
outer  segment  of  the  cone  of  muscles  surrounding  the  optic 
foramen.  In  the  deeper  portions  of  the  orbit  the  external 
is  seen  on  section  to  be  the  largest  of  the  group  (Fig.  6). 
As  it  passes  forward  towards  the  upper  and  outer  portion 
of  the  globe,  its  transverse  section  presents  an  ellipse,  the 
longer  diameter  being  from  above  downward.  On  the  inner 
side  it  is  in  contact  with  the  orbital  fat,  and  on  the  outer 
side  with  the  bone. 

When  a  transverse  section  is  made  of  the  orbit,  5  or  6 
millimeters  from  the  globe  (Fig.  7),  we  find  that  the  rela- 
tions of  the  external  rectus  have  changed  somewhat.  It  is 
now  smaller  than  'near  its  origin,  more  circular  in  shape, 
and  is  surrounded  on  all  sides  by  fat,  being  at  this  point 
about  on  a  line  with  the  optic  nerve. 

The  sixth  nerve  passes  along  the  inner  surface  of  this 
muscle,  which  it  supplies,  and  posteriorly  the  ophthalmic 
artery  lies  close  to  its  upper  and  inner  portion ;  farther  for- 
ward the  lacrymal  artery  often  courses  along  its  upper  edge, 
and  anteriorly,  near  its  insertion,  it  is  in  relation  above  with 
the  lacrymal  gland. 

The  principal  insertion  is  in  form,  position,  and  size 
nearly  the  counterpart  of  the  opposing  muscle — the  inter- 
nal rectus.  The  insertion  of  the  external  is  convex  toward 
the  cornea,  the  center  of  the  line  being  near  the  hori- 
zontal plane  of  the  eye.  The  external  rectus  is,  however, 
inserted  farther  back  than  the  internal,  the  center  being  on 
the  average  not  far  from  7  mm.  from  the  corneal  edge. 

Occasionally  one  end  of  this  line  is  farther  from  the  edge 


The  Extraocular  Muscles 


of  the  cornea  than  the  other  end,  the  variations  being  about 

equal.     The  secondary  insertions  of  the  external  rectus  fol- 

low the  same  general  plan  as  those  of  the  internal.     At 

each  edge  of  the  tendon, 

above  and  below,  we  usu- 

ally find  filaments  of  con- 

nective tissue.     These  are 

also  seen  connecting  the 

ocular     surface     of      the 

muscle  near  its    insertion 

with      the      capsule       of 

Tenon,  and  from  the  ex- 

ternal flat   surface  of  the 

insertion  fibers  of  connec- 

tive tissue  bend  upward, 

and     especially    outward, 

to  blend  with  other  fibers, 

forming  part  of  the  fascia 

orbito-ocularis  or  external 

check  ligament. 

The  following  meas- 
urements have  been  made 
Of  this  muscle  by  Schnel-  FIG.  13.—  External  rectus.  External 


The    fascia    orbito-ocularis  is 

reflected  ljght  with  a  fold  of 

connective  tissue  passing  from  the  exter- 
nal surface  of  the  muscle  to  the  margin  of 
the  orbit. 


ler:    length,    45.8    mm.;    surface. 
breadth,  9.2  mm. ;    thick- 
ness, 1.6  mm. 

The    relation    between 
the  thickness    of    the   in- 
ternal rectus  muscle  and   that   of   the  external  has  been 
studied  accurately,  and  exact  measurements  of  this    rela- 
tion have  been  made  by  Fuchs,  Volkmann,  and  Schneller. 

The  plan  adopted  was  to  make  transverse  sections  of  these 
muscles  through  their  fleshy  portions  and  measure  the  num- 
ber of  square  millimeters  of  surface  thus  presented  by  the 
divided  ends.  As  a  result  Schneller  found  that  the  internal 
rectus  measured  39  square  millimeters,  while  the  external 
rectus  measured  26,  giving  a  relation  between  the  two 
of  about  TOO  to  66.6.  Thus,  although  the  fleshy  part 
of  the  externus  is  broader  from  above  downward  than  the 


24  The  Superior  Rectus 

internus,  the  latter  muscle,  being  thicker  in  a  horizontal 
direction,  contains  a  larger  number  of  square  millimeters  in 
section.  The  weight  of  the  two  muscles  is  about  the  same, 
according  to  Adachi,  being  for  the  internal  rectus  0.57 
gram  and  for  the  external  0.56  gram. 

It  is  interesting  also  to  observe  to  what  degree  the 
lateral  recti  are  shortened  when  the  eye  turns  in  or  out. 
This  has  been  estimated  with  considerable  exactness  by 
Schneller.  (B  19.)  When  the  eye  makes  an  excessive  rota- 
tion, turning  inward  45  degrees  there  is  a  shortening  of  the 
internal  rectus  of  about  23.5  per  cent,  of  its  length,  but 
when  it  turns  outward  40  degrees  from  the  contraction  of 
the  rectus  externus,  there  is  a  shortening  of  about  16.75 
per  cent. 

§  11.  The  Superior  Rectus  arises  along  the  upper 
edge  of  the  two  openings  or  figure  of  eight  from  which 
the  other  muscles  spring.  The  curve  of  its  origin  has 
its  concavity  downwards,  some  of  the  filaments  commenc- 
ing also  from  the  division  between  these  two  foramina. 
The  muscle  is  somewhat  ribbon-shaped,  except  near  its 
origin,  where  it  is  more  rounded.  A  cross-section  of  the 
orbit  near  the  apex  (Fig.  7)  shows  this  muscle,  thin  and 
flattened,  lying  at  the  upper  and  inner  portion,  touching 
internally  the  levator  palpebrae,  and  having  its  upper  edge 
only  in  contact  with  the  roof  of  the  orbit.  More  anteriorly 
we  find  that  its  position  has  changed  somewhat.  The  long 
diameter  is  now  more  nearly  horizontal  (Fig.  8).  It  is  in 
contact,  by  its  inner  and  upper  surface,  with  the  levator 
palpebrae;  below  and  inward  it  rests  upon  the  fat,  while  its 
external  edge  is  in  contact  with  the  frontal  nerve.  Still 
farther  forward  the  long  diameter  of  its  section  is  almost 
horizontal,  and  the  levator  now  rests  upon  and  covers  the 
superior  rectus.  The  superior  orbital  nerve  lies  upon  this 
muscle  near  its  outer  edge,  and  usually  also  the  ophthalmic 
artery  and  vein  and  the  ophthalmic  branch  of  the  fifth 
nerve. 

The  principal  insertion  of  the  superior  rectus  is  in  the  form 
of  an  arc,  somewhat  convex  anteriorly,  the  center  of  the 
line  being  rather  to  the  inner  side  of  the  vertical  meridian  of 


The  Extraocular  Muscles  25 

the  eye.  It  is  usually  7  to  8  mm.  from  the  edge  of  the 
cornea.  The  length  of  the  line  of  insertion  is  on  the  average 
not  far  from  10.5  mm.,  its  inner  end  being  decidedly  nearer 
the  cornea  than  the  outer.  The  secondary  insertions 
are  well  marked  (Fig.  14).  There  are,  as  usual,  not  only 
the  fibers  at  each  edge  of  the  tendons,  but  also  fibers  pass- 
ing from  the  ocular  surface  near  its  insertion  to  the  capsule 
of  Tenon,  and  other  fibers  passing  from  the  upper  flat  sur- 
face to  the  adjacent  tissue.  These  fibers  are  especially 


FIG.  14. — Ocular  surface  of  superior  rectus  and  levator. 

abundant  near  the  outer  edge  of  the  tendon,  where  they 
blen'd  with  the  network  that  holds  the  lacrymal  gland  in 
place. 

The  following  measurements  have  been  made  of  the 
muscle:  length,  41.8  mm.;  width,  9.2  mm.;  thickness, 
1.6  mm. ;  in  section,  1 1.3  square  mm. ;  weight,  0.51  gram. 

The  structure  of  the  superior  rectus  is  of  interest  because, 
as  already  observed,  it  contains  in  its  epimysium  and  also  in 
its  perimysium  a  greater  abundance  of  elastic  connective 


26  The  Inferior  Rectus 

tissue  fibers  than  is  found  in  any  other  part  of  the  body,  or 
even  in  any  of  the  other  ocular  muscles.  (B  25.) 

§  12.  The  Inferior  Rectus  arises  from  the  space  between 
the  optic  foramen  and  the  opening  for  the  motor  oculi. 
At  first  the  muscle  is  round,  but  almost  at  once  it  flattens, 
so  that  its  section  forms  an  ellipse  (Figs.  8  and  9),  and  lies 
in  the  lower  part  of  the  orbit,  somewhat  external  to  the 
optic  nerve.  Above  and  to  the  outer  side  it  is  covered  by 
fat,  and  internally  it  is  almost  in  contact  with  the  internal 
rectus. 

When  seen  farther  forward  it  is  more  nearly  below  the 
optic  nerve  and  is  entirely  surrounded  by  fat;  farther  for- 
ward still,  the  relations  are  practically  the  same,  though, 
as  it  approaches  the  globe,  it  flattens  out  yet  more,  the  long 
diameter  being  horizontal  as  it  passes  towards  its  insertion. 
The  primary  insertion  of  the  inferior  rectus  is  in  a  line 
always  convex  to  the  margin  of  the  cornea.  The  center  of 
this  curve  is  usually  near  the  vertical  plane  of  the  globe, 
though,  in  occasional  instances,  the  center  of  the  insertion 
lies  a  little  to  the  outer  side  of  that  point. 

The  average  distance  of  the  center  of  the  primary  inser- 
tion from  the  cornea  is  about  5.5  mm.,  and  its  length  is 
from  9.8  to  10.3  mm.,  the  inner  end  of  the  line  being  almost 
invariably  nearer  to  the  cornea  than  the  outer. 

The  secondary   insertions  of  the  inferior  rectus  are  in- 
teresting and  sometimes  im- 
portant,   clinically.     At  each 
edge  of  the  tendon  there  are 
the  usual  small  bands  of  con- 
nective  tissue,    more  or    less 
marked,  and  the  ocular  face  of 
Fir,.   is.-Vertical  section  of  in-       the  tendon  js  also  connected, 
ferior  oblique   and    inferior    rectus,  .  .  -11 

showing  secondary  connective  tissue  "Gar  lts  insertions,  With  the 
attachments  to  the  inferior  oblique.  outer  surface  of  the  Capsule 

of    Tenon.     Very    important 

secondary  attachments  also  bind  the  sheath  of  the  inferior 
rectus  to  that  of  the  inferior  oblique,  as  can  be  seen  after 
using  selective  stains  for  connective  tissue.  The  abundance 
and  general  direction  of  these  fibers  are  imperfectly  shown  in 


The  Extraocular  Muscles  27 

the  accompanying  illustration  of  a  vertical  section  (Pig.  15). 
Other  fibers  from  the  lower  surface  of  the  muscle  pass  later- 
ally and  forward  to  blend  with  fibers  from  the  inferior 
oblique,  all  of  which,  curving  around  the  eyeball,  have  been 
called,  as  we  shall  see  later,  '  the  suspensory  ligament.'  The 
measurements  of  the«inferior  rectus  given  by  Volkmann  are 
as  follows:  length,  40  mm.;  in  section,  15.85  square  mm. ; 
weight,  0.67  gram. 

§  13.  The  Superior  Oblique  comes  from  the  apex  of 
the  orbit,  at  the  upper  and  inner  side  of  the  ring  of 
muscular  fibers  which  surround  the  optic  foramen.  Indeed, 
the  fibers  of  this  muscle,  at  its  origin,  lie  so  close  to  the 
superior  and  internal  recti  that  there  is  usually  some  diffi- 
culty in  separating  them.  Near  its  origin  it  is  noticeably 
smaller  than  the  superior  and  internal  rectus.  As  it  passes 
forward  it  increases  in  size,  being  somewhat  fusiform,  but 
as  it  approaches  the  pulley  it  gradually  decreases  in  bulk,  at 
the  same  time  becoming  tendinous,  and  when  it  reaches  that 
point  it  is  hardly  more  than  three  or  four  millimeters  in 
diameter.  After  passing  through  the  pulley  it  is  reflected 
outward  and  backward,  to  be  inserted  into  the  sclerotic, 
near  the  upper  portion  of  the  upper  and  outer  quadrant  of 
the  posterior  half  of  the  globe. 

Between  the  pulley  and  the  point  of  insertion  the  charac- 
ter of  the  muscle  is  entirely  changed.  It  no  longer  looks 
like  a  muscle,  but  is  tendinous  in  character,  and  as  the 
fibers  spread  out,  fine  and  glistening,  they  resemble  rather 
an  aponeurotic  covering  of  the  globe.  Or,  let  us  consider 
its  course  more  in  detail. 

When  we  examine  a  frontal  section  of  the  orbit,  made 
near  its  apex,  the  superior  oblique  is  not  found  in  the  upper 
and  inner  corner,  as  we  might  expect,  but  about  on  a  level 
with  the  optic  nerve,  at  the  extreme  inner  edge  of  the  orbit. 
(Fig.  6.)  In  this  position  it  lies  close  to  the  bone.  Above 
it  are  the  superior  rectus  and  the  levator,  while  below  and 
externally  to  it,  is  the  inferior  rectus.  When  a  transverse 
section,  made  a  little  farther  in  front  of  this,  is  examined, 
the  appearance  differs  but  slightly  from  the  last.  The 
muscle  in  section  is  larger  than  before,  triangular  in  form, 


28  The  Superior  Oblique 

rests  upon  bone,  and  is  in  the  same  relation  to  the  muscles 
just  mentioned,  except  that  the  internal  rectus  at  this  point 
has  moved  relatively  nearer  to  the  optic  nerve.  (Fig.  7.) 
When  another  transverse  section,  near  the  equator  of  the 
globe,  is  examined,  the  tendon  of  the  superior  oblique  is  seen 
still  lying  close  to  the  bone  and  covered  by  connective  tissue, 
which  here  begins  to  thicken  to  form  the  pulley,,  (Fig.  9.) 
When  the  tendon  of  this  muscle  reaches  its  farthest  point 
at  the  upper  and  inner  angle  of  the  orbit  it  is  still  more  re- 
duced in  size,  and  passes  through  that  remarkable  anatomi- 
cal structure, — the  pulley  of  the  superior  oblique.  This 
consists  of  successive  bands  of  connective  tissue,  which, 
starting  from  a  small  projection  on  the  bone  at  this  point  of 
the  orbit,  pass  outward  to  form  almost  a  perfect  loop  and 
then  return  to  the  point  of  origin,  each  loop  being 
strengthened  by  other  bands  and  fibers  of  similar  tissue. 
Sections  of  the  pulley  both  in  a  horizontal  and  vertical  direc- 
tion which  were  made  to  ascertain  whether  a  bursa  exists  at 
this  point,  gave  only  negative  results.  As  the  muscle  passes 
through  this  pulley  it  changes  its  course,  as  already  men- 
tioned, turning  outward  and  backward  at  an  angle  varying 
from  fifty  to  about  fifty-six  degrees  from  the  frontal  plane. 
Vertical  sections  through  the  orbit  show  that  the  tendon  of 
the  muscle  is  round  as  it  comes  out  of  the  pulley,  but  almost 
immediately  it  flattens  from  above  downward  and  in  passing 
as  just  mentioned  to  the  insertion  into  the  sclerotic, 
spreads  out  like  a  fan  of  thin,  white,  glistening  fibers.  The 
relations  of  this  muscle  are  not  especially  important,  al- 
though it  is  crossed  posteriorly  by  the  ophthalmic  artery 
and  more  anteriorly  by  the  supraorbital  branches  of  that 
vessel  with  their  corresponding  veins.  The  direction  of  the 
line  of  the  primary  insertion  tends  to  a  curve  more  or  less 
complete,  the  center  of  which  corresponds  roughly  to  the 
pulley.  But  this  is  not  always  the  case.  Sometimes  the 
arc  is  irregular,  or  at  intervals  straight  or  wavelike  in 
form.  These  variations  are  so  marked  that  Fuchs  (B  15) 
would  divide  them  into  two  types.  In  the  first,  the  line 
of  attachment  is  a  long  one  extending  backward  from  about 
the  center  of  the  upper  and  outer  quadrant  to  the  pos- 


The  Extraocular  Muscles 


29 


terior  half,  so  that  the  curve  is  convex  posteriorly.  In  some 
cases  nearly  half  of  the  line  is  on  the  median  side  of  the  ver- 
tical meridian.  The  other  type  of  the  line  of  insertion  is 
quite  different.  In  this,  the  end  of  the  line  which  is  nearest 
to  the  cornea  begins  in  the  upper  and  outer  quadrant  of  the 
posterior  half  of  the  globe  two  or  three  millimeters  behind 
the  equator;  the  line  then  runs  almost  straight  backward  at 


FIG.  16. — Tendon  of  the  superior  oblique,  showing  its  primary 
insertion  and  the  lateral  secondary  insertions.  The  one  on  the 
left  side  is  particularly  well  marked. 

an  angle  of  about  forty-five  degrees  with  the  vertical  plane, 
and  within  three  or  four  millimeters  of  the  point  where  that 
plane  would  cut  the  globe.  This  line  of  insertion  is  either 
straight,  or,  as  already  stated,  slightly  convex.  Between 
the  two  types  there  are  great  variations  in  detail,  not  only 
as  to  the  length  of  the  line  and  the  degree  of  its  convexity, 
but  also  as  to  the  angle  which  the  central  fibers  of  this 


3O  The  Inferior  Oblique 

reflected  portion  of  the  muscle  make  with  the  base  of  the 
cornea. 

The  secondary  insertions  are  numerous,  and  may  be 
divided  into  two  groups.  To  the  first  belong  those  small 
filaments  which  pass  from  the  muscles  to  the  adjoining 
structures  before  the  muscle  passes  through  the  pulley. 
To  the  second  group  belong  the  delicate  offshoots  which 
pass  from  the  reflected  tendon  in  various  directions.  These 
are  especially  numerous  at  the  edges  of  the  tendon  near  its 
principal  insertion;  indeed,  as  the  fan-like  fibers  spread  out, 
it  is  very  difficult,  on  most  globes,  to  determine  where  the 
primary  insertion  ends  and  where  the  secondary  ones  begin. 
(Fig.  16.)  It  is  this  difficulty,  doubtless,  that  accounts  for 
the  different  lengths  of  insertion  given  by  different  anato- 
mists. There  are  also  small  fibers  which  pass  from  the 
upper  flat  surface  of  the  tendon  to  the  structures  above, 
and  still  others  which  can  easily  be  seen  when  the  tendon 
is  divided  near  the  pulley  and  turned  backward  from  the 
globe.  From  the  pulley  to  its  insertion,  this  muscle  meas- 
ured, in  an  emmetropic  eye,  18. 5  millimeters  and  the  breadth 
at  its  insertion — that  is,  the  length  of  the  line — was  16.5  mil- 
limeters. Very  decided  variations  in  this  respect,  however, 
are  often  noticed. 

§  14.  The  Inferior  Oblique. — This  is  the  only  one  of  the 
extraocular  muscles  which  does  not  arise  at  the  apex  of  the 
orbit.  If  we  slide  the  finger  along  the  orbital  plate  of 
the  superior  maxilla  we  find  a  shallow  depression  situated 
near  its  anterior  border,  not  far  from  the  lacrymal  bone. 
This  depression  is  some  five  or  six  millimeters  in  length, 
not  quite  so  wide,  and  just  deep  enough  to  be  perceived  by 
the  finger.  From  this,  the  inferior  oblique  takes  its  origin. 

Its  general  direction  is  outward  and  upward,  and  at  such 
an  angle  that  it  appears  to  rise  quite  abruptly.  By  the  time 
it  has  reached  the  line  of  the  inferior  rectus  its  upper  sur- 
face is  in  contact  with  the  lower  surface  of  that  muscle,  and 
then,  curving  upwards  and  backwards,  it  passes  to  its  inser- 
tion. (Fig.  17.) 

The  relations  of  this  muscle  are  best  seen  in  frontal  sec- 
tions of  the  orbit.  When  we  examine  such  a  section,  made 


The  Extraocular  Muscles  31 

slightly  posterior  to  the  margin  of  the  cornea,  the  inferior 
oblique  is  seen  curving  upward  toward  the  globe,  surrounded 
by  fat  above  and  below.  If  another  frontal  section  is 
made,  nearer  the  center  of  the  globe,  we  find  the  form  and 
position  of  the  inferior  oblique  has  changed  decidedly. 
Here  it  is  much  more  flattened  than  near  its  origin,  its 
upper  surface  is  in  contact  with  the  inferior  rectus,  while 
below  it  rests  on  the  orbital  fat,  and  is  supported  also 
by  an  unusual  amount  of  connective  tissue,  forming  a 


FlG.  17. — Eye  in  its  normal  position  in  the  orbit  (Merkel  and  Kallius). 

part  of  the  suspensory  ligament  of  Lockwood.  From 
thence  it  curves  up  and  turns  backward  to  its  point  of 
insertion. 

The  primary  insertion  is  in  the  outer  and  lower  quadrant 
of  the  posterior  portion  of  the  globe.  Its  form  is  that  of  a 
slight  curve,  with  the  concavity  directed  somewhat  down- 
ward and  forward.  The  average  length  of  this  curve  is  from 
about  7  to  9.5  mm.,  although  in  some  cases  it  measures  10 
to  12  mm. 


32  The  Muscles  of  the  Eye 

Nor  is  the  position  always  the  same.  Its  posterior  end 
usually  begins  2  or  3  mm.  from  the  optic  nerve,  but  this 
varies  greatly,  being  dependent  somewhat  on  the  length  of 
the  eye. 

This  muscle,  like  the  others,  is  covered  with  compara- 
tively little  connective  tissue  near  its  origin,  but,  as  it  ad- 
vances, the  fibrous  covering  increases  in  thickness  and  gives 
off  small  attachments  to  adjacent  structures.  Beneath  the 
inferior  rectus  numerous  fibers  are  given  off  posteriorly  and 
superiorly,  connecting  the  sheaths  of  the  two  muscles. 
Near  this  point  also,  other  fibers  are  directed  anteriorly 
to  form  part  of  the  fascia  orbito-ocularis,  and  especially 
the  suspensory  ligament  of  Lockwood.  As  the  muscle 
then  advances  to  its  principal  insertion,  it  gives  off  fibers 
to  the  globe  and  also  a  few  which  go  to  the  connective 
tissue  of  the  orbit,  though  these  are  usually  small  and  some- 
times absent.  Finally,  there  are,  at  each  edge  of  the  primary 
insertion,  the  usual  accessory  fibers  which  cause  this  inser- 
tion to  merge,  apparently  by  imperceptible  gradations,  into 
the  adjacent  connective  tissue. 

§  15.  Measurements  of  the  Primary  Insertions  and 
Methods  of  Recording  the  Results  Obtained. — As  there 
will  be  occasion  to  refer  frequently  to  the  exact  position  of 
the  muscles,  it  is  desirable  to  know  how  the  landmarks  of 
the  globe  can  be  most  conveniently  located,  and  measure- 
ments made  from  them  recorded  accurately  for  comparison. 
After  an  eye  has  been  removed  and  inflated  or  injected,  it 
is  turned  so  that  when  placed  on  the  table  it  lies  in  the  same 
position,  relatively,  that  it  occupied  in  the  head.  This  is 
not  always  easy,  especially  for  the  novice,  but  a  little  prac- 
tice and  observation  of  the  lines  of  insertion  will  enable  one 
to  become  quite  expert.  Another  difficulty  in  measuring 
the  insertions  is  to  obtain  fixed  lines  from  which  to  begin. 
The  edge  of  the  cornea,  it  is  true,  serves  that  purpose  fairly 
well,  although  not  exactly,  as  the  clear  portion  merges 
gradually  into  the  opaque.  But  for  measurements  of  the 
recti,  and  especially  of  the  oblique,  it  is  desirable  to  deter- 
mine their  position  with  reference  to  the  equator,  and  to 
the  vertical  and  horizontal  planes.  This  apparently  small 


The  Extraocular  Muscles 


33 


matter  is  nevertheless  one  of  importance,  and  as  no  mention 
of  the  point  could  be  found  in  the  literature,  and  marking 
the  globe  with  pen  or  pencil  could  not  be  done  at  all 
satisfactorily,  a  simple  method  was  devised  which  serves  the 
purpose  perfectly. 

This  consists  in  slipping  over  the  globe,  thread-like  bands 
of  india-rubber  to  mark  the  equator  and  the  vertical  and 
horizontal  planes.  From  these  lines  measurements  can  be 
made  with  a  readiness  and  an  exactness  which  are  eminently 
satisfactory. 

In  recording  the  attachments  of  the  ocular  muscles  it  is 
convenient  to  represent  the  curves  of  the  insertion  in  the 


YS 


VI 


FIG.  18. — Lines  of  insertion  of  one  eye  plotted  to  a  scale.  A  A,  line  of  the 
cornea.  B  B,  line  of  the  equator.  I,  line  where  a  horizontal  plane  cuts  the 
inner  portion  of  the  globe.  E,  line  where  a  horizontal  plane  cuts  the  external 
portion  of  the  globe.  VS,  line  where  a  vertical  plane  cuts  the  superior  portion 
of  the  globe.  VI,  line  where  a  vertical  plane  cuts  the  inferior  portion  of  the 
globe. 

same  manner  as  lines  on  the  earth  are  shown  in  the  ordinary 
Mercator's  projection,  and  to  do  this,  as  Fuchs  has  shown, 
on  a  scale  at  least  four  times  the  size  of  the  globe  itself.  If, 
therefore,  the  diameter  of  the  normal  eye  is  23  millimeters, 
the  length  of  the  equator  is  72.2,  and  the  line  on  which  it 
is  plotted  is  288.8  millimeters.  In  Fig.  18  A  A  is  the  line 
representing  the  edge  of  the  cornea,  and  parallel  to  it  is  the 
equator  B  B.  It  is  true  that  the  cornea  is  not  perfectly 
circular,  but  its  representation  by  a  straight  line  is  suf- 
ficiently exact  for  this  purpose. 


34 


The  Primary  Insertions 


Finally,  the  four  perpendiculars  are  the  representations, 
by  the  Mercator  projection,  of  the  lines  in  which  the  globe 
is  cut  by  the  horizontal  and  vertical  planes.  From  these 
various  lines  it  is  possible  to  locate  the  position  of  the  vari- 
ous muscular  insertions.  If  the  separate  plottings  of  dif- 
ferent eyes  be  drawn  successively  on  the  same  paper,  we 
have  at  a  glance  the  variations  which  they  present. 

§  1 6.  The  Primary  Insertions  Considered  as  a  Whole. 
— In  order  to  ascertain  the  direction  of  each  muscle,  and 
therefore  its  line  of  traction,  it  is  desirable  to  study  each 
one  separately,  as  we  have  done.  But  if  we  would  make 
comparisons  of  the  insertions  we  must,  of  course,  consider 
them  together.  The  results  of  measurements  already 
quoted  are  therefore  given  below,  and  to  these  a  few  other 
data  can  be  added  (B  24).  From  these  it  appears  that 
the  average  distance  of  the  line  of  insertion  of  the  recti 
from  the  edge  of  the  cornea  as  found  by  different  ob- 
servers is  as  follows: 

FUCHS.      WEISS.     ADACHI.    HOWE. 

Internal  rectus 5.5 

Inferior  rectus 6.5 

External  rectus 6.9 

Superior  rectus 7.7 


WEISS. 

5.85 
6.85 

6.75 
8.01 


5-5 
6.8 

7-3 

8.3 


5-7 
6.7 
7-4 
7-6 


These  measure- 
ments vary  some- 
what according  to 
whether  the  eye 
be  normal  or  short- 
ened (H)  or  length- 
ened (M).  It  should 
be  observed,  how- 
ever,  that  in 
emmetropiathe  in- 
sertions are  in 
the  line  of  a  spiral. 
(Fig.  20.)  Com- 
mencing with  the 
internal  rectus, 
which  is  nearest  to 


R.S. 


R.L 


R.M. 


R.I. 

FIG.    20. — Diagram   showing   that   the   insertions 
are  approximately  in  the  line  of  a  spiral. 


the  cornea,  this  line  curves  down,  out,  and  up  to  the  inser- 


The  Primary  Insertions 


35 


tion  of  the  superior  rectus,  which  is  seven  and  a  half  or  eight 
millimeters  distant  from  it. 

Again,   the   length   of  these  lines   (the   breadth   of  the 
tendon)  is  given  as  follows : 

FUCHS.  WEISS.      ADACHI. 

Internal  rectus 10.3  10.76 

Inferior  rectus 9.8  10.35 

External  rectus 9.2  9.67 

Superior  rectus 10.6  10.75 

It  may  be  of  interest  to  glance  at  the  variations  in  the 
position,  length,  and  form  of  the  lines  of  primary  insertions 

Rijht 


9.9 

9.0 

8.4 

IO.O 


FIG.  21. — Lines  of  insertion  of  several  eyes  plotted  together  in  order  to 
show  the  differences  in  the  form  and  place  of  the  insertion.  In  this  figure  all 
of  the  right  eyes  are  grouped  together  in  the  upper  portion,  and  the  left  eyes 
in  the  lower  portion.  In  both  C  represents  the  line  of  the  cornea,  E  the 
equator  of  the  globe. 

which  have  been  found  to  exist  in  a  series  of  twenty-one  eyes. 
Instead  of  drawing  each  of  these  sets  of  lines  on  a  separate 
set  of  meridians,  they  are  all  placed  upon  a  single  sheet, 
thus  making  comparisons  possible  at  a  glance.  (Fig.  21.) 


36  The  Primary  Insertions 

This  plan  represents,  as  has  been  said,  the  projection  of 
the  globe  on  a  plane  surface,  like  the  Mercator  map.  "Int  " 
is  the  line  in  which  a  horizontal  plane  would  cut  the  globe 
on  its  inner  side,  and  crossing  this,  of  course,  are  the  in- 


FIG.  22. — The  lines  of  insertion  of  all  the  muscles  in  the  average  eye. 
Modified  by  additional  measurements  from  the  figure  of  Merkel  and  Kallius. 
M,  internal  rectus  (medialis).  L,  external  rectus  (lateralis).  S,  superior  rec- 
tus.  I,  inferior  rectus.  OS,  superior  oblique.  OI,  inferior  oblique.  No. 
I  shows  the  right  eye  seen  from  above,  No.  2  seen  from  the  median  plane, 
No.  3  from  below,  No.  4  from  the  outer  side. 

sertions  of  the  different  internal  recti.  "Ex"  shows  the 
line  in  which  a  horizontal  plane  would  cut  the  globe  on  the 
outer  side,  and  crossing  this  are  the  insertions  of  the  different 
external  recti,  etc. 

While  it  is  evident  that  very  decided  variations  occur  in 
different  individuals  in  the  insertion  of  some  of  the  muscles, 
especially  of  the  oblique,  an  examination  of  the  measure- 


The  Secondary  Insertions  37 

ments  shows  that  the  insertions  of  the  recti,  especially  of 
the  internus,  are  remarkably  regular.  By  taking  the  average 
of  the  measurements  in  a  considerable  number  of  eyes  we  are 
able  to  represent  graphically  the  position  which  the  different 
muscles  assume  when  we  regard  the  globe  from  four  differ- 
ent points  of  view — namely,  from  above,  from  the  median 
side,  from  below,  and  from  the  temporal  side.  These 
are  seen  in  the  accompanying  Fig.  22,  Nos.  I,  2,  3,  4. 
Schneller  has  shown  (Arch.  f.  Ophth.,  1890,  3,  s.  160)  that 
it  is  possible  to  measure  the  position  of  the  line  of  insertion 
of  the  internus  or  of  the  other  recti  while  making  several 
of  the  ordinary  operations.  He  goes  so  far  as  to  say  that  in 
tenotomy  with  displacement  backwards  (Riicklagerung) 
we  may  lay  bare  the  conjunctival  surface  of  the  tendon 
for  six,  eight,  or  even  ten  mm.  with  no  detriment  to  the 
patient ;  or,  when  advancement  is  to  be  made,  for  a  distance 
of  10-13  mm. — that  is,  from  a  fifth  to  a  quarter  of  the 
entire  length  of  the  internus. 

Although  we  will  have  occasion  later  to  question  the 
advisability  of  such  precedures,  undoubtedly  it  would  be 
better  if  surgeons  would  take  more  pains  thus  to  observe 
the  position,  direction,  and  extent  of  the  line  of  insertion  of 
the  tendons  of  the  recti  when  they  are  divided,  for  in 
many  cases  the  difficulties  for  which  we  operate  are  un- 
doubtedly dependent  on  abnormal  insertions. 

§  17.  The  Secondary  Insertions. — In  studying  these 
dissections  we  have  already  learned  not  only  what  the  sec- 
ondary insertions  are,  but  their  general  arrangement.  It 
remains,  however,  to  classify  them  more  definitely  for  future 
reference.  As  these  fibers  stretch  out  in  all  directions  from 
the  muscles  near  their  insertions  they  may  be  conveniently 
divided  into  four  groups.  The  first  forms  a  continuation 
of  one  edge  of  the  tendon,  being  the  lateral  secondary  inser- 
tion. For  example,  in  the  case  of  the  internal  rectus,  we 
have  the  superior  lateral  secondary,  or  in  the  case  of  the 
superior  rectus,  the  external  lateral  secondary  insertion, 
etc. 

A  part  of  this  group  includes  those  attachments  at  the 
other  edge  of  the  tendon ;  with  the  internal  rectus,  we  have 


38  The  Secondary  Insertions 

the  inferior  lateral  secondary,  or  with  the  superior  rectus, 
we  have  the  internal  lateral  secondary  insertion. 

These  two  groups  are  evidently  similar  to  each  other, 
being  simply  on  opposite  edges  of  the  main  tendon.  They 
are  easily  seen  by  drawing  out  the  muscle  at  right  angles 


FIG.  19. — Transverse  (frontal)  section  anterior  to  the  equator  magnified 
three  times  (Hans  Virchow).  This  illustration  is  especially  interesting,  as  it 
gives  a  sectional  view  not  only  of  the  tendons  (T)  at  this  point,  but  also  of  the 
connective  tissue  fibers  (A)  extending  from  one  tendon  toward  the  next. 
These  are  the  lateral  secondary  insertions  seen  in  section.  Merkel  calls 
these  fibers  the  adminiculum  tendinis.  At  the  point  where  this  section  is 
made,  the  capsule  of  Tenon  (Ca)  lies  entirely  external  to  the  tendons.  The 
clinical  importance  of  the  arrangement  of  the  secondary  insertions  at  this  point 
is  clear,  especially  their  relation  to  the  various  forms  of  tenotomy  and  of 
advancement. 

to  the  globe.  These  lateral  attachments  are  mentioned  first 
because  they  are  so  easily  seen  and  because  their  clinical 
importance  is  so  evident. 

A  second  set  of  fibers  constitute  what  may  be  termed  the 
ocular  secondary  insertions.  These  pass  from  the  ocular 


,  The  Secondary  Insertions  39 

surface  of  the  muscle  either  to  the  capsule  of  Tenon,  or 
directly  to  the  sclerotic.  If  care  has  been  taken  not  to 
break  these  small  fibers,  they  can  be  seen  extending  quite 
far  back  toward  the  equator.  They  appear  as  minute  white 
fibers  stretching  from  the  muscle  to  the  capsule,  but  occa- 
sionally assuming  an  oblique  direction. 

These  secondary  insertions  are  quite  easily  seen  in  trans- 
verse and  vertical  section^  of  the  globe,  though  it  is  very 
difficult  to  complete  the  cutting  and  mounting  without 
disturbing  the  relation  of  the  muscle  and  connective  tissue 
fibers. 

Finally,  there  is  a  third  group  of  fibers  which  pass  from 
the  muscle  near  its  insertion  outward  in  the  direction  of  the 
orbit  and  are  therefore  the  orbital  secondary  insertions. 
These  are  easy  to  demonstrate,  and  they  can  be  seen  best, 
perhaps,  when  dissecting  the  anterior  portion  of  the  muscle 
from  behind,  as  described  on  page  7.  If  the  globe  be 
drawn  backward,  these  fibers  can  be  easily  seen  stretching 
from  the  muscles  off  in  the  direction  of  the  margin  of  the 
orbit.  It  is  not  always  possible  to  follow  individual  fibers 
directly  from  the  muscle  to  the  bone,  for  they  blend  with 
other  portions  of  the  fascia  orbito-ocularis,  forming  in- 
ternally and  externally  a  part  of  that  tissue  which  we  shall 
study  presently  as  the  check  ligaments. 

When  examined  in  thin  horizontal  sections,  these  secon- 
dary orbital  insertions  appear  to  pass  in  general  forward  and 
outward.  Those  fibers  which  arise  at  some  distance  pos- 
teriorly to  the  principal  insertion  are  for  the  most  part 
directed  somewhat  forward ;  those  nearer  to  the  insertion, 
forward  and  outward,  while  the  foremost  fibers  pass  outward 
and  even  backward. 

The  arrangement  of  the  secondary  insertions  of  the 
obliques  is  in  general  similar  to  that  of  the  recti.  The 
lateral  secondary  insertions  are  particularly  well  marked, 
and  as  the  tendon  of  the  superior  oblique,  for  example,  is 
lifted  away  from  the  globe  it  is  usually  quite  impossible  to 
determine  any  one  point  on  either  side  which  separates  the 
primary  insertion  from  those  lateral  expansions  of  connec- 
tive tissue  which  constitute  the  secondary  insertions.  We 


40  Connective  Tissue  of  the  Orbit 

see  simply  a  broad,  glistening  band  which,  being  attached 
to  the  globe  in  its  central  portion,  gradually  thins  out  into 
the  connective  tissue  of  the  capsule  of  Tenon  on  either  side. 
(See  Fig.  16.)  The  ocular  secondary  attachments  are  also 
well  marked,  being  similar  to  those  which  connect  the  recti 
to  the  outer  portion  of  the  capsule  of  Tenon,  but  neither 
the  superior  nor  the  inferior  oblique  can  be  said  to  have 
secondary  orbital  insertions  in  the  manner  in  which  that 
term  applies  to  the  recti,  and  such  fibers  of  connective  tissue 
as  do  pass  outward  from  these  muscles  in  the  direction  of 
the  orbit  are  of  no  clinical  importance. 

§  1 8.  General  Plan  of  the  Arrangement  of  the  Con- 
nective Tissue  of  the  Orbit,  Especially  the  Capsule  of 
Tenon. — When  describing  the  secondary  insertions  of  the 
recti,  it  was  necessary  to  refer  to  the  capsule  of  Tenon  and 
the  check  ligaments.  So  much  confusion  exists  in  regard 
to  them,  and  their  clinical  importance  is  so  great,  that  we 
must  understand  exactly  what  is  meant  by  those  terms,  and 
incidentally  glance  at  the  general  arrangement  of  the  con- 
nective tissue  within  the  orbit.  It  is  impossible  to  consider 
all  the  varying  descriptions  of  the  so-called  "capsule  of 
Tenon"  given  by  different  anatomists,  or  by  ophthalmolo- 
gists who  evidently  were  not  anatomists. 

Suffice  it  to  say,  that  when  Tenon  described,  about  a 
hundred  years  ago,  what  he  called  a  "new  tunic  of  the 
eye,"  that  description  covered  a  considerable  part  of  all  the 
connective  tissue  in  the  orbit.  Since  then,  different  anato- 
mists have  made  and  described  different  dissections  of  this 
"tunic,"  for  we  must  remember  that  it  is  impossible,  with 
the  naked  eye,  to  decide  whether  the  hair-like  fibers  which 
we  see  extending  in  various  directions  are  connective  tissue 
fibers  or  not,  and  that  only  an  examination  under  higher 
magnification  than  is  possible  with  the  dissecting  micro- 
scope will  determine  their  nature. 

The  description  of  the  connective  tissue  of  the  orbit 
which  is  most  familiar  to  French  and  English  readers,  is  the 
one  given  by  Motais  (B  48)  and  copied  from  him  by  Mad- 
dox,  Landolt,  and  other  writers  on  the  muscles.  These 
colored  illustrations  are  very  beautiful,  and  such  schematic 


Connective  Tissue  of  the  Orbit  41 

representations  are  easily  understood,  but  it  must  be  re- 
membered that  they  are  only  diagrams.     (Fig.  23.) 

Another  description  of  the  connective  tissue  of  the  orbit 
is  given  by  Merkel  and  Kallius  in  the  last  edition  of  Graefe- 
Saemisch,  and  one  still  more  complete  by  Hans  Virchow 
(B  5 1),  which  we  have  already  mentioned.  But  these  descrip- 
tions differ  in  detail,  for  the  fact  is  that  dissections  as  seen 
with  the  naked  eye  or  made  with  the  dissecting  micro- 
scope lead  to  different  results,  and  therefore  different 
descriptions. 


FIG.  23. 

FIG.  23. —  Schematic  representation  of  the  check  ligaments  according  to 
Motais.  Connective  tissue  in  red,  muscles  in  black. 

Very  recently,  however,  several  selective  stains  for  con- 
nective tissue  have  been  found,  and  one  or  two  of  these 
have  assisted  materially  in  demonstrating  the  arrangement 
of  those  fibers  near  the  insertion  of  the  recti,  although,  un- 
fortunately, the  difficulty  in  photographing  colors  makes  it 
impossible  to  show  the  results  except  by  drawings.  After 


42  Connective  Tissue  of  the  Orbit 

some  experimenting  with  these  stains  of  connective  tissue,  I 
found  that  some  of  the  most  satisfactory  results  could  be 
obtained  with  a  process  in  which  potassium  permanganate 
was  an  important  factor.  A  description  of  this  method  and 
of  the  arrangement  of  the  connective  tissue  was  published 
in  1902.' 

It  tends  to  clearness  to  understand  thus  the  reason  for 
the  difference  in  descriptions,  and  also  where  the  most  recent 
ones  can  be  found.  For  our  present  purpose,  when  taking 
only  a  general  view  of  the  arrangement  of  the  connective 
tissue  in  the  orbit,  it  is  well  to  view  it,  as  has  been  done  by 
Motais  and  earlier  writers,  as  arranged  in  three  layers.  The 
first  and  most  external  of  these  is  the  periosteum.  The 
anterior  portion  of  this,  which  is  connected  with  the  fascia 
in  that  vicinity,  is  the  only  part  which  concerns  the  student 
of  the  ocular  muscles,  and  that  only  slightly. 

The  second  layer,  or  the  cone  of  the  connective  tissue  in 
the  orbit,  is  that  which  may  be  said  to  envelop  the  muscles 
as  it  passes  from  one  to  the  other.  Motais  has  given  some 
very  beautiful  drawings  of  this,  but  these  diagrams  do  not 
show  the  other  fibers  of  the  connective  tissue  which  radiate 
in  every  direction,  especially  toward  the  opening  of  the 
orbit,  and  form  a  network  supporting  all  the  tissues.  If  a 
section  be  made  behind  the  globe  parallel  to  the  plane  of 
the  opening  of  the  orbit,  we  see  not  only  the  four  recti 
in  section,  but  also  the  connective  tissue  surrounding  each. 
The  fibers  extend  from  the  edge  of  one  muscle  in  each 
direction  toward  the  adjoining  muscle,  thus  forming  almost 
a  band,  and  in  some  of  the  illustrations  given  by  Motais  this 
looks  like  a  tubular  sheath  in  the  orbit,  consisting  of  two 
layers  between  which  lies  the  muscles.  But  that  appearance 
is  deceptive,  for  similar  fibers  stretch  out  also  in  every  direc- 
tion, forming  a  network  throughout  the  entire  orbit,  and  the 
bands  which  go  from  one  muscle  to  another  are  only  a  part 
of  this  entire  framework.  The  deeper  portion  of  this  second 
layer  is  evidently  rather  of  academic  interest.  But  the 
fibers  near  the  opening  of  the  orbit,  where  they  separate  to 

1  The  Connective  tissue  of  the   Orbit.      Prize   Essay  Medical  Society  of  the 
State  of  New  York,  1902. 


Connective  Tissue  of  the  Orbit 


43 


J2  x  c    '••-*- 

C     OJ  TO    *••     w 

5  «  «  .2?-?!  -5 


6  .2  5  i-  .2 

liilki 


44 


form  the  fascia  orbito-ocularis  and  the  check  ligaments,  are 
of  much  clinical  importance. 

In  order  to  show  these  fibers  and  the  general  arrangement 
of  the  connective  tissue  near  the  opening  of  the  orbit,  three 

diagrams  are 
given,  one  show- 
ing a  vertical 
section  (Fig.  23 
'"  Motais),  another 
an  enlarged  hori- 
zontal section 
(Fig.  24,  Hans 
Virchow),  and 
another  a  hori- 
zontal section  on 
a  smaller  scale 
(Fig.  25).  The 
last  gives  a  di- 
agrammatic view 
of  these  connec- 
tive tissue  fibers 
near  the  front  of 
the  orbit. 

The    third    or 

innermost  layer  of  connective  tissue  is  more  exactly  and 
properly  the  capsule  of  Tenon.  This  layer,  commencing 
posteriorly,  where  the  optic  nerve  enters  the  orbit,  covers 
that  nerve  and  then  passes  forward  upon  the  globe.  It 
is  separated  from  the  sclerotic  by  a  space  barely  per- 
ceptible to  the  naked  eye.  Across  this  space  fibers  pass 
from  the  capsule  to  the  globe,  holding  the  two  together, 
yet  allowing  the  globe  to  rotate  within  the  capsule  as  in  a 
ball-and-socket  joint.  The  interstices  of  this  space  are  occu- 
pied by  a  lymphatic  network  well  described  by  Schwalbe. 
(B  52.)  Where  the  innermost  layer  covers  the  globe  proper 
it  constitutes  the  true  "capsule  "  of  Tenon.  The  concave 
surface  of  this  capsule  can  be  seen  when  a  vertical  section  is 
made  through  the  globe,  and  it  is  allowed  to  fall  out  of  its 
enclosing  capsule.  (Fig.  26.) 


FIG.  25. — Schematic  illustration  of  a  horizontal  sec- 
tion of  the  globe.  This  shows  the  arrangement  of 
the  connective  tissue  fibers  on  both  sides  of  the  in- 
ternal and  external  recti. 


Capsule  of  Tenon  45 

A  question  which  has  been  discussed  frequently  and  at 
length  is  just  how  far  forward  the  capsule  of  Tenon  extends 
— that  is,  whether  it  ends 
posterior  to  the  insertion  of 
the  recti  muscles,  or  anterior 
to  it,  and  therefore  whether 
the  muscles  perforate  the  cap- 
sule in  order  to  reach  the 
globe.  As  a  result  we  have 
different  opinions  and  in- 
genious theories  in  relation  to 
tenotomies  and  operations  for 
advancement.  The  fact  is 
that  these  theories  are  due  FIG.  26.— Vertical  section  of  the 

largely      tO      a      difference       of    globe.      The  latter  has  fallen  out  of 

methods  in  demonstrating  the  its  enclosins  caPsule-  and  is  held  s"*- 

.      .         ..  .         ,          pended  by  the  conjunctiva  anteriorly 

anatomical   detai  s,  and    also      A  K    ,v,  i 

and    by    the    external    rectus   muscle 

to   a   difference    of   terms    in  (y?/)  (Merkel  and  Kallius). 
describing  what  is  then  seen. 

For,  as  the  tendon  of  the  muscle  passes  to  its  insertion  in 
the  sclerotic  it  is  supported  by  connective  tissue  fibers  on 
all  sides.  If,  therefore,  these  fibers  are  considered  to  be- 
long to  the  globe,  the  capsule  may  be  said  to  be  perforated 
by  the  tendon,  but  if  the  fibers  are  considered  to  belong  to 
the  muscle — that  is,  to  be  offshoots  from  it,  or  secondary 
attachments, — then  the  muscle  may  be  said  to  blend  with 
the  sclerotic  anterior  to  the  capsule. 

§  19.  Fascia  Orbito-Ocularis  and  the  Check  Liga- 
ments.—  Having  thus  reviewed  the  general  arrangement  of 
the  connective  tissue  inside  of  the  orbit,  let  us  study  more 
carefully  that  important  portion  at  the  opening  of  the  orbit, 
known  as  the  fascia  orbito-ocularis,  with  the  check  ligaments. 

This  fascia  may  be  compared  to  a  curtain  stretched  across 
the  front  of  the  orbit  through  which  the  globe  of  the 
eye  partly  protrudes.  The  fibers  extend  from  the  entire 
edge  of  the  orbit  toward  the  globe,  lying  in  general  just 
beneath  the  ocular  conjunctiva.  They  are  abundant  and 
thick  in  some  localities,  helping  to  form  the  check  liga- 
ments, and  scanty  where  the  fascia  is  thin. 


46  The  Check  Ligaments 

The  fascia  orbito-ocularis  is  easy  to  demonstrate.  If  we 
complete  the  dissection  already  outlined,  to  a  point  where 
there  remains  only  a  ring  of  bone  around  the  edge  of  the 
orbit,  and  the  globe  be  then  suspended  by  the  optic  nerve, 
an  excellent  idea  is  obtained  of  the  fascia.  Moreover, 
if  this  fascia,  when  thus  stretched  out,  is  examined  more 
closely,  it  will  be  observed  that  at  certain  parts,  especially 
in  the  vicinity  of  the  internal  and  external  recti,  the  con- 
nective  tissue  fibers  are  thicker  and  more  abundant.  These 
are  the  check  ligaments. 

Another  and  very  instructive  method  of  examining  the 
check  ligaments  is  by  means  of  transmitted  light.  If, 
with  the  globe  suspended,  the  specimen  is  held  up  before 
a  window,  it  will  be  observed  that  while  the  entire  fascia 
is  translucent,  certain  portions  of  it,  especially  in  the 
vicinity  of  the  recti,  are  thick  and  firm.  The  best  view  is 
obtained  in  a  dark  room  by  placing  a  small  electric  bulb 
near  to  the  cornea,  and  as  the  light  shines  through  the  fascia, 
the  extent  and  form  of  the  check  ligaments  are  seen  with 
special  distinctness.  When  the  entire  specimen  is  sus- 
pended the  check  ligaments,  being  extended,  are  thin  and 
long  (Fig.  27),  but  when  the  bony  edge  of  the  orbit  is  ap- 
proached to  the  globe,  so  as  to  leave  less  space  between 
them,  the  thickened  part  of  the  fascia  is  somewhat  quadri- 
lateral in  shape,  its  longer  side  resting  on  the  bone.  (Fig. 
28.) 

Let  us  now  look  more  closely  at  three  or  four  portions 
of  the  fascia  orbito-ocularis — namely,  internally,  externally, 
below,  and  above  the  globe, — which  have  been  called  the 
check  ligaments. 

The  internal  check  ligament  is  that  irregular  quadnlatetal 
thickened  portion  of  the  fascia  which  passes  from  the  median 
surface  of  the  internal  rectus  to  the  crista  lacrymalis.  Its 
ocular  attachment  is  continuous  with  the  connective  tissue 
which  envelops  the  globe  more  or  less  completely,  and 
which  constitutes  the  anterior  part  of  the  capsule  of  Tenon. 
Indeed,  the  fibers  of  the  connective  tissue  interlace  so  com- 
pletely that  the  ocular  portion  of  the  fascia  orbito-ocularis 
is  really  the  same  as  the  capsule  itself.  The  median  end  of 


The  Check  Ligaments 


47 


the  check  ligament  is  attached  along  the  posterior  border 
of  the  crista  lacrymalis  for  a  distance  of  ten  or  twelve  milli- 


FlG.  27. — The  fascia  orbito-ocularis  extended  and  viewed  by  transmitted 
light.  This  illumination  is  produced  by  placing  an  electric  light  behind  the 
connective  tissue  bands. 


FIG.  28. — The  fascia  orbito-ocularis  lighted  from  behind.  The  connective 
tissue  fibers  are  relaxed  by  allowing  the  globe  to  approach  nearer  the  edge  of 
the  orbit. 

meters.  This  median  limit  of  the  internal  check  ligament 
is  therefore  well  defined.  Above  and  below,  however,  the 
fibers  of  the  ligament  blend  gradually  with  the  other  por- 
tions of  the  fascia  orbito-ocularis. 


48 


The  Check  Ligaments 


The  external  check  ligament  is  made  up  of  the  thickened 
bands  of  the  fascia  orbito-ocularis  which  pass  from  the  ex- 
ternal rectus  to  the  margin 
of  the  orbit.  Its  limits  are 
not  quite  as  clearly  defined 
as  those  of  the  internal 
check  ligament.  Still,  this 
thickened  portion  is  suf- 
ficiently well  marked  to  form 
also  a  quadrilateral  band,  the 
outer  margin  of  which  is  at- 
tached to  the  margin  of  the 
orbit,  and  the  inner  edge  to 
the  connective  tissue  on  the 
outer  surface  of  the  external 
rectus.  Toward  the  upper 
and  lower  edges  of  this  ligament  the  fibers  blend  with  the 
adjacent  parts  of  the  fascia  orbito-ocularis. 

The  inferior  check  ligament  is  another  part  of  the  fascia 


FIG.  29. — Photograph  of  the  inner 
wall  of  the  orbit.  The  attachment  of 
the  internal  check  ligament  is  along 
the  center  of  this  black  line. 


M.  rectus  sup.  et  lev.  palp.  sup. 


FIG.  30. — Schematic  representation  of  the  fascia  at  the  opening  of  the  orbit. 
The  upper  edge  of  the  orbit  has  been  removed  to  show  the  band  of  fascia 
which  passes  from  its  inner  and  upper  edge  to  the  globe  (Merkel  and  Kallius). 

orbito-ocularis,  which  is  particularly  thickened  over  the  in- 
sertion of  the  inferior  rectus.  In  this  case  the  general 
direction  of  the  fibers  is  not  from  the  globe  toward  the  edge 


The  Check  Ligaments  49 

of  the  orbit,  as  is  the  case  with  the  two  check  ligaments 
already  described,  but  below  the  globe  the  fibers  pass  rather 
in  the  direction  of  the  inferior  oblique  muscle — in  fact,  they 
form  its  anterior  covering.  Moreover,  they  tend  to  arch 
around  the  lower  portion  of  the  globe,  and  for  that  reason 
they  received  long  ago  from  Lockwood  (B  346)  the  name 
of  the  ligamentum  suspensorium  oculi. 

The  superior  check  ligament  has  also  been  described  as  a 
separate  part  of  the  same  fascia.  It  is  true  that  the  fibers 
of  the  connective  tissue  in  the  vicinity  of  the  superior  rectus 
are  also  thickened,  and  to  that  portion  the  name  check 
ligament  may  be  applied,  but  the  limits  of  this  ligament  are 
not  so  well  defined  as  are  those  of  the  other  thickened  parts 
of  the  fascia.  In  reality,  however,  the  fascia  at  this  point, 
being  connected  firmly  to  the  orbit  and  to  the  superior 
rectus,  does  also  constitute  a  check  upon  the  rotation  of  the 
globe,  and  in  that  sense  it  forms  a  real  check  ligament. 

Mention  of  the  check  ligaments  will  recall  to  the  reader 
the  illustrations  and  diagrams  by  Motais  already  given.  It 
will  be  remembered  that  a  horizontal  section  of  the  internal 
check  ligament,  for  example,  is  represented  as  similar  to  the 
letter  Y,  one  arm  being  attached  to  the  edge  of  the  orbit, 
the  other  arm  to  the  globe,  their  junction  being  continuous 
with  the  surface  of  the  muscle.  While  this  is  true  in  a 
rough  way,  we  must  remember  that  the  outer  arm,  for  ex- 
ample, is  not  formed  of  a  single  firm  band,  but  of  other 
fibers  of  connective  tissue  also  springing  from  the  orbit 
somewhat  farther  back  than  the  main  line  of  origin.  Nor  is 
there  any  single  group  of  fibers  which  come  in  one  small 
band  from  the  globe  as  represented  in  transverse  section  by 
the  internal  arm  of  the  letter  Y,  as  this  portion  is  made  up, 
instead,  of  numerous  bands  of  fibers,  already  described  as 
the  orbital  secondary  insertion  of  the  recti. 

§  20.  Surgical  Anatomy  of  the  Check  Ligaments. — 
Before  leaving  this  part  of  the  subject  let  us  take  up  one  or 
two  questions  concerning  the  check  ligaments  which  relate 
rather  to  surgical  than  to  descriptive  anatomy.  First: 
Are  these  entitled  to  the  name  check  ligaments?  Yes, 
any  one  can  satisfy  himself  of  this  who  will  take  the 


The  Check  Ligaments 


trouble  to  make  the  suitable  dissections.  If,  after  the 
dissection  is  almost  completed,  the  cornea  be  rotated  toward 
the  nose  by  traction  on  the  internal  rectus,  a  point  is  soon 
reached  where  the  motion  is  impeded  by  tension  made 
on  these  ligaments;  if,  then,  the  ligament  be  divided,  the 
cornea  can  then  be  turned  much  farther  inward.  In  fact, 
with  sufficiently  free  division  of  the  ligament,  the  position 
of  the  cornea  can  be  almost  reversed. 

Again,  exactly  to  what  extent  does  this  act  as  a  check  in 
the  lesser  movements?  A  very  careful  study  of  this  ques- 
tion has  been  made  by  Motais, 
and  his  conclusions  are  copied  in 
most  of  the  text-books.  Briefly 
stated,  they  are  to  the  effect  that 
when  a  muscle  begins  to  contract 
it  makes  little  or  no  traction  upon 
the  orbital  arm  of  the  "Y  "  liga- 
ment, but,  as  the  contraction 
becomes  greater,  it  draws  more 
and  more  upon  that  end  of  the 
Y,  until  the  latter  becomes  so 
tense  that  it  does  not  permit  any 
f  u  rther  contraction.  After  making 
the  dissection  described  by  Merkel  and  drawing  on  first  the 
internal  and  then  the  external  rectus,  it  seemed  that  the 


FIG.  31.  —  Check  ligaments 
when  the  eye  is  in  the  primary 
position.  Modified  from  Motais. 


FIG.  32.  —  Check  ligaments  during 
partial  contraction  of  the  external  rec- 
tus.  Modified  from  Motais. 


FIG.  33.  —  Check  ligaments  during 
extreme  contraction  of  the  external 
rectus.  Modified  from  Motais. 


changes    in   the    position  of    the  check    ligament     should 
not    be    represented   as   they  are    usually,  but    as    in  the 


Supernumerary  Muscles  51 

accompanying  diagrams  modified  from  Motais.  (Figs.  31, 
32,  and  33.) 

Finally,  how  far  can  the  check  ligaments  be  extended? 
Inasmuch  as  muscles  like  the  recti  in  other  portions  of  the 
body  can  sometimes  contract  about  one  half  their  length,  we 
would  naturally  expect  the  maximum  contraction  of  the  in- 
ternal rectus,  for  example,  to  rotate  the  center  of  the  cornea 
inwards  through  an  arc  corresponding  to  half  the  length 
of  that  muscle.  In  practice,  however,  we  know  that  this 
is  out  of  the  question,  the  difference  being  partly  due  to  the 
restraining  action  of  the  check  ligaments. 

§21.  Supernumerary  Muscles  of  the  Orbit— So  little 
mention  is  ordinarily  made  of  the  supernumerary  muscles 
that  one  not  accustomed  to  dissections  may  be  surprised 
perhaps  that  such  muscles  exist.  But  any  one  who  has  ob- 
served the  variations  in  the  secondary  insertions  in  different 
individuals,  or  who  knows  how  the  bands  of  connective 
tissue  in  different  parts  of  the  orbit  vary  in  their  distribu- 
tion, can  easily  understand  how  these  bands  may  be  suffi- 
ciently marked  in  certain  cases  to  be  described  as  special 
muscles. 

For  it  must  be  understood  that  none  of  these  super- 
numerary muscles  approaches  in  size  the  other  extrinsic 
muscles,  but  that  they  consist  only  of  bundles  of  striped 
muscular  fibers  interwoven  with  connective  tissue  fibers. 

I.  Muscle  of  Homer. — Among  these  supernumerary  mus- 
cles, attention  should  first  be  called  to  the  muscle  of 
Homer.  This  is  seldom  mentioned  in  descriptions  of 
the  contents  of  the  orbit,  yet  it  is  often  of  considerable  im- 
portance in  connection  with  the  operation  for  tenotomy  of 
the  internal  rectus.  Although  described  first  by  an  Ameri- 
can, W.  E.  Homer  (B  64),  one  of  the  best  representations 
of  it  is  given  by  Hans  Virchow  (Fig.  24)  in  the  monograph 
already  referred  to  (B  51).  The  muscle  consists  of  a  few 
bands,  which  arise  from  the  crista  lacrymalis  posterior  and 
pass  horizontally  forward  and  somewhat  outward,  to  be  in- 
serted into  the  tissue  just  anterior  and  to  the  inner  side  of 
the  caruncle.  Farther  anteriorly,  they  pierce  the  network 
of  connective  tissue  to  be  inserted  into  the  conjunctiva  and 


52  Supernumerary  Muscles 

the  adjacent  structures.  The  function  of  this  muscle  is  not 
well  understood.  Possibly  it  is  the  remnant  of  the  band 
which  moves  the  nictitating  membrane  in  the  lower  ani- 
mals, or  it  may  assist  in  facilitating  the  flow  of  tears  away 
from  the  globe.  However  that  may  be,  the  direction  of 
the  fibers  shows  without  question  that  it  also  tends  to  draw 
the  conjunctiva  inwards  and  backwards  as  long  as  the  orbital 
fascia  is  in  its  natural  position.  While  the  action  of  Homer's 
muscle  is  thus  difficult  to  see  in  the  normal  condition,  yet 
when  the  fascia  is  disturbed  to  any  great  extent,  as  in  lacera- 
tions such  as  follow  certain  forms  of  tenotomy,  the  fibers 
of  this  muscle,  then,  having  nothing  to  counteract  them, 
draw  the  conjunctival  tissue  in  and  backward,  and  we  have 
a  sinking  of  the  caruncle,  with  the  consequent  deformity. 

2.  The  Gracillimus  or  Transversus. — At  quite  an  early  date 
careful  anatomists  recognized  at  least  one  or  two  other 
supernumerary  muscles — these  lying  near  the  roof  of  the 
orbit.  Albinus  was  probably  the  first  to  describe  one  of 
them.  He  found  a  band  passing  from  the  levator  with  con- 
nections inward,  especially  to  the  superior  oblique,  and  this 
muscle  he  called  the  gracillimus. 

Another  description  of  the  gracillimus,  or  of  a  band  similar 
to  it,  is  in  an  article  by  Bochdalek  (B  70).  He  found  this 
additional  muscle  in  both  orbits  of  one  subject.  These 
differed  slightly  from  the  gracillimus  described  by  Albinus, 
and  also  differed  somewhat  from  each  other. 

A  more  careful  search  for  the  gracillimus,  or  a  muscle 
similar  to  it,  was  made  by  Budge  (B  63)  of  Greifswald.  He 
says  "While  making  preparations  of  the  muscles  of  the  eye, 
I  found  a  muscular  portion  given  off  from  the  levator  palpe- 
brae  superior.  This  portion  branched  into  two  small  bun- 
dles, from  the  inner  side  of  that,  muscle,  and  then,  passing 
inward,  they  were  inserted  into  the  trochlearis. "  He  ex- 
amined about  twenty  orbits  of  children  and  grown  persons, 
as  to  this  point,  and  there  were  only  five  without  some 
traces  of  these  fibers,  although  in  some  instances  they 
appeared  to  be  hardly  more  than  threads.  Among  the 
muscular  bands  found  by  Bochdalek  in  the  upper  portion 
of  the  orbit  he  describes  another  as  the  anomalous  trans- 


Supernumerary  Muscles 


53 


versus.  (Fig.  34.)  He  says  it  arises  from  the  anterior  and 
upper  portion  of  the  orbital  plate  of  the  ethmoid,  and 
passes  almost  directly  across  the  upper  part  of  the  orbit. 
At  its  origin  it  con- 
sists of  small  tend- 
inous bands  3  to  4 
mm.  in  thickness; 
these,  enlarging  in- 
to fleshy  bundles, 
give  off  various  at- 
tachments to  neigh- 
boring fascia,  and 
especially  to  the 
levator  palpebrae. 
In  fact,  when  the 
transversus  is  small 
it  forms  practically  FlG  34._Supernumerary  muscle,  the  transver- 

a  part  of  that  mus-  Sus  (Tr.). 
cle. 

Other  supernumerary  bands,  more  or  less  abundant  in 
muscular  fibers,  have  been  met  with  in  the  orbit,  and  are 
mentioned  in  the  literature  from  time  to  time  (B  54  to  72), 
some  of  the  writers  being  apparently  ignorant  of  the  obser- 
vations made  by  others.  It  is  quite  certain,  therefore,  that 
these  additional  fibers  are  more  common  than  the  usual 
descriptions  would  lead  us  to  expect.  Even  in  a  small  col- 
lection of  orbits  there  are  usually  one  or  two  which  illus- 
trate very  well  the  presence  of  these  minute  muscular 
bands.  Usually,  these  are  what  might  be  termed  support- 
ing fibers,  which  are  more  or  less  continuous  with  the  muscle, 
and  which  run  in  the  same  direction.  Sometimes,  however, 
they  are  quite  distinct,  passing  in  the  direction,  not  of  the 
fleshy  portion  of  the  muscle,  but  rather  of  its  tendinous  ex- 
pansions. A  good  example  of  this  is  found  in  the  striated 
fibers  often  present  in  the  upper  and  outer  portions  of  the 
orbit,  in  the  region  where  the  external  bands  of  the  levator 
bend  outward  or  even  partly  backward,  to  be  lost  in  the 
tissue  of  the  orbit  near  the  lacrymal  gland. 

Although  we  are  considering  at  present  only  questions  of 


54  Heredity 

anatomy,  we  may  turn  for  a  moment  to  the  probable  func- 
tions of  these  fibers.  If  we  judge  of  the  action  of  a  muscle 
from  its  point  of  origin  and  insertion,  we  must  infer  that  the 
larger  of  these,  the  gracillimus  or  transversus,  assists  the 
levator, — as,  for  example,  in  the  act  of  winking. 

§  22.  Influence  of  Heredity  on  the  Ocular  Muscles. — 
Recent  studies  show  more  and  more  the  tendency  of  the  off- 
spring to  resemble  the  parent  even  in  the  most  minute  de- 
tails of  structure.  This  is  particularly  noticeable  in  certain 
parts  of  the  eye.  Perhaps  one  of  the  best  examples  is  in  the 
curvature  of  the  cornea.  We  find  occasionally  in  parent  and 
child  an  astigmatism  with  the  degree  and  the  axis  wonder- 
fully similar.  When  we  remember  that  this  means  a  repro- 
duction in  the  child  of  the  curvature  of  the  parent's  cornea 
even  to  a  fraction  of  a  millimeter,  we  begin  to  realize  the 
far-reaching  effects  of  this  law  of  heredity.  A  compara- 
tively slight  consideration  of  the  subject  leads  one  to  think 
also  that  we  have  not  given  to  this  subject  the  attention  it 
deserves,  when  studied  from  the  standpoint  of  the  ocular 
muscles.  It  is  a  common  observation  to  find  that  there  is 
an  hereditary  tendency  to  certain  forms  of  heterophoria. 
Most  practitioners  also  have  met  with  families  in  which 
there  are  two,  three,  or  more  cases  of  abnormal  convergence. 
This  same  tendency  is  shown  in  the  transmission  through 
successive  generations  of  peculiar  types  of  nystagmus  and 
especially  certain  forms  of  ocular  paralyses.  Apparently 
no  one  has  thus  far  made  systematic  measurements  of 
the  attachments  of  the  recti  in  such  cases,  but  inasmuch  as 
different  members  of  squinting  families  often  apply  to  the 
same  surgeon,  and  as  it  is  possible  to  make  quite  accurate 
measurements  of  the  primary  insertions  of  the  recti  imme- 
diately after  certain  forms  of  tenotomy,  it  would  seem  that 
this  question  might  be  easily  studied.  In  this  connection 
we  are  reminded  of  the  contention  made  years  ago  by 
Stevens,  that  races  and  individuals  who  have  long,  tall 
skulls  have  also,  as  a  rule,  a  tendency  to  quite  a  different 
position  of  the  visual  axes  than  those  who  have  short,  broad 
skulls,  and  that  these  positions  of  the  axes  necessitate  ab- 
normal traction  of  certain  muscles.  It  is  probable  that 


Heredity  55 

some  such  tendencies  do  exist.  But  as  the  conclusions 
thus  far  reached  are  based  on  apparently  insufficient  meas- 
urements, it  still  remains  for  others  to  study  this  question 
and  the  general  influence  of  heredity  on  the  ocular  muscles. 


CHAPTER  II. 

THE  INTRAOCULAR  MUSCLES  AND  OTHER  STRUCTURES 
CONCERNED  IN  ACCOMMODATION. 

§  I.  The  Intraocular  Muscles. — In  most  chapters  on 
the  ocular  muscles  comparatively  little  attention  has  been 
paid  to  the  act  of  accommodation.  The  more  one  studies 
the  subject,  however,  the  more  evident  does  it  become  that 
the  structures  involved  in  this  act  are,  from  the  clinical 
point  of  view,  quite  as  important  as  the  recti  and  obliques. 
The  intraocular  and  the  extraocular  groups  are  as  essen- 
tial to  each  other  as  are  the  blades  of  a  pair  of  scissors. 
The  fact  that  the  ocular  muscles  have  been  viewed  in  former 
monographs  almost  entirely  from  the  standpoint  of  the  ex- 
traocular group  was  one  of  the  reasons  for  undertaking  this 
study. 

In  order  to  understand  more  exactly  the  mechanism  of 
accommodation  it  is  necessary  also  to  recall  some  anatomical 
details  relating  to  the  structures  by  which  that  act  is 
accomplished. 

(A)  The  Ciliary  Muscle. — When  an  antero-posterior  sec- 
tion of  the  ciliary  region  is  viewed  under  even  slight  magni- 
fication, it  is  easy  to  see  that  the  muscular  fibers  may  be 
divided  into  three  groups,  as  was  first  described  by  Iwanoff 

(B  79). 

The  outer  or  meridional  layer  is  composed  of  fibers  which 
lie  next  to  the  sclerotic,  and  whose  general  direction  is 
almost  parallel  to  the  latter.  They  appear  to  arise  near 
the  periphery  of  the  iris,  and  passing  backward  close  to  the 

56 


The  Intraocular  Muscles 


sclerotic  become  continuous  posteriorly  with  the  choroid 
at  and  beyond  the  ora  serrata.  In  the  center,  this  layer  of 
fibers  is  about  a  millimeter  in  thickness,  but  it  is  thinner 
posteriorly,  where  it  becomes  flattened  into  a  delicate  mesh 
interwoven  with  the 
choroid.  In  other 
words,  we  may  con- 
sider the  fibers  of 
this  layer  as  at- 
tached anteriorly  to 
the  periphery  of  the 
iris  and  to  the  scle- 
rotic, and  posterior- 
ly to  the  choroid. 

The  second,  or 
radial  portion  lies 
more  internally. 
The  fibers,  viewed 
in  section,  are  seen 
to  spring  from  the 
same  point  ante- 
riorly as  those  of 
the  outer  layer,  to 
pass  backward  not 
quite  so  nearly  con- 
centric with  the 
curve  of  the  scle- 
rotic, but  spreading 
out  toward  the  cen- 
ter of  the  eye  in 
the  shape  of  a  fan 

whose  external  edge  is  longer  than  the  internal  one. 
(Fig.  35.)  There  is  no  line  of  demarcation  between  the 
middle  group  of  fibers  and  those  external  or  internal  to 
it.  The  size  and  extent  of  this  portion  vary.  In  the 
hypermetrope,  the  fibers  pass  more  directly  backward, 
while  in  the  myope  they  are  more  nearly  parallel  with 
the  fibers  in  the  first  group — namely,  the  meridional 
fibers.  The  radial  fibers  of  the  ciHaiy  muscle  do  not 


FIG.  35. — Transverse  section  of  the  ciliary  mus- 
cle. (A)  emmetropia,  (B)  myopia,  and  (C )  hyper- 
metropia.  (Iwanoff.) 


The  Muscles  of  the  Eye 


appear  to  have  as  definite  an  origin  as  do  those  in  the  first 
or  outer  group.  They  interlace  with  each  other,  pass  back- 
ward, and  at  their  posterior  extremities  give  attachment  to 
the  fibers  of  the  zone  of  Zinn. 

The   third  group  of  fibers   which   compose   the   ciliary 
muscle  consists  of  circular  fibers  lying  just  within  the  radial 

group.  It  is  gen- 
erally known  as 
the  muscle  of 
Miiller  or  of  Bow- 
man, but  Wallace 
of  New  York  (B 74) 
was  apparently 
one  of  the  first  to 
demonstrate  these 
fibers  and  the  cili- 
ary  muscle  as 
a  whole.  When 
these  circular  fi- 
bers are  viewed  in 
cross-section  they 
appear  as  mere 
points.  In  the 
hypermetropic  eye 
these  points  are 
numerous,  but  in 
the  myopic  eye, 
especially  in  the 
higher  degrees, 
Iwanoff  considers 
that  they  are 
lacking  entirely. 
These  three  groups  of  fibers  which,  taken  together,  we  call 
the  ciliary  muscle,  regulate  the  accommodative  changes  of 
the  crystalline  lens,  as  we  shall  see  later,  and  in  doing  so 
perhaps  contribute  more  to  our  comfort  and  enjoyment 
than  any  other  muscle  of  the  body. 

(B)  The  Dilator  Pupillae. — This  interesting  portion  of  the 
iris  should  not  be  passed  without  mention,  although  hardly 


FIG.  36. — Dilator  pupillae.    Antero-posterior  sec- 
tion.    Magnification  250.     (Verhoeff.) 


The  Intraocular  Muscles 


59 


more  than  that  is  possible  here.  Its  existence  was  estab- 
lished in  the  earlier  half  of  the  last  century,  and  although 
many  later  students  could  not  confirm  the  observation,  we 
know  now  that  this  was  due  largely  to  the  irregular  course  of 
the  fibers.  One  of  the  best  articles  on  this  muscle  is  by  Gru- 
nert  (B  94),  which  gives  also  quite  a  complete  bibliography  of 
the  subject.  When  an  attempt  is  made  to  reproduce  any 
of  these  sections  by  photography,  the  half-tone  represen- 
tation does  not  do  justice  to  the  specimen.  Figure  36  is 
sufficient  to  give  a  general  idea  of  the  position  and  direction 


FIG.  37. — Vertical  section  through  iris  and  ciliary  region.  Sphincter 
pupillse  (s).  The  dots  representing  the  fibres  in  cross-section  seem  to 
extend  quite  a  distance  from  the  pupillary  margin.  This  is  because  of 
the  considerable  magnification  used.  Stroma  of  the  iris  (str). 


of  the  fibers  of  the  dilator.  The  section  is  made  from  be- 
fore backwards.  The  portion  on  the  left  is  the  stroma  of 
the  iris  (facing  the  cornea).  That  on  the  right  is  the 
epithelium  of  the  iris  (facing  the  lens).  Between  the  stroma 
and  the  epithelium  there  can  be  seen  the  dark  line  of  fibers 
which  together  constitute  the  dilator  of  the  pupil.  With 
stronger  magnification  the  separate  fibers  of  the  muscle 
can  be  recognized  as  if  they  were  the  spokes  of  a  wheel 
whose  hub  is  the  pupil.  The  radiating  fibers  do  not  pass 
straight  off  toward  the  periphery  of  the  iris,  however,  but 
interlace  with  adjoining  fibers  in  a  more  or  less  irregular 
network. 

(C)  The  Sphincter  Pupillae  consists  of  a  band  of  circular 
fibers  which  is  situated  in  the  iris  near  the  edge  of  the  pupil 


60  .  Zonula  Zinnii 

and  concentric  with  it.  These  fibers  lie  in  the  stroma  of 
the  iris,  just  at  its  pupillary  margin,  considerably  nearer  to 
the  posterior  than  to  the  anterior  surface.  They  are  seen  in 
section  in  Fig.  37  (s).  An  examination  of  the  sphincter  under 
rather  high  power  shows  that  the  different  parts  lie  close 
together  and  are  interwoven,  each  bundle  of  fibers  being 
supplemented  by  others  near  to  it,  so  that  together  they 
act  as  a  single  muscle  to  contract  the  pupil. 

§  2.  Suspensory  Ligament  of  Zinn — Zonula  Zinnii. — The 
foregoing  consideration  of  both  the  extraocular  and  intra- 
ocular muscles  might  perhaps  be  deemed  sufficient  for  our 
purpose.  But  the  important  act  of  accommodation  is  greatly 
modified  by  what  we  may  call  the  "resistance  "  offered  to  the 
ciliary  muscle  by  any  rigidity  of  the  lens,  by  any  astigma- 
tism present,  or  by  other  imperfections  of  the  refracting 
media.  If,  therefore,  we  wish  to  have  a  proper  appreciation 
of  the  work  done  by  the  intraocular  muscles  we  must  see  in 
what  this  so-called  "resistance"  consists.  In  other  words, 
we  must  consider,  at  least  briefly,  the  manner  in  which  the 
ciliary  muscle  is  attached  to  the  lens,  we  must  glance  at  its 
structure,  its  imperfections  —  especially  in  the  production 
of  astigmatism — and  other  causes  of  "resistance"  which 
may  exist. 

This  zonula  is  usually  described  as  arising  from  a  certain 
special  part  of  the  ciliary  process,  and  then  dividing  into  two 
layers,  one  of  which  passes  over  the  anterior  and  the  other 
over  the  posterior  surface  of  the  lens.  It  was  supposed  that 
these  two  layers  compressed  both  sides  of  the  lens,  or  relaxed 
their  pressure  equally  in  the  changes  of  accommodation. 
More  recent  studies,  however,  have  shown  that  the  struc- 
ture of  the  ligament  is  not  so  simple,  nor  do  the  sides  of 
the  lens  change  equally.  When  we  examine  the  zonula 
in  section  we  find  it  to  be  composed  of  very  fine  fibers  which 
are  either  transparent  or  highly  refractive.  They  do  not 
arise  from  any  one  part  of  the  ciliary  process,  but  along  its 
entire  inner  aspect.  The  fibers  which  arise  from  the  part 
nearest  to  the  iris  pass  almost  directly  to  the  anterior  sur- 
face of  the  lens.  Those  which  arise  more  posteriorly, — that 
is,  from  the  ends  of  the  more  distant  radial  fibers, — also 


Zonula  Zinnii 


61 


FIG.  38. — Vertical  section  through  the  ciliary  region  (Retzius).  The  fibers 
which  constitute  the  zone  of  Zinn  (d)  stretch  from  the  ciliary  processes 
to  the  lens.  The  figure  shows  the  direction  and  arrangement  of  these  fibers; 
also  the  manner  in  which  the  posterior  fibers,  especially,  have  secondary  at- 
tachment fibers,  as  in  (a). 


FIG.  39. — Diagrammatic  view,  from 
posterior  surface,  of  the  insertion  of  the 
zone  of  Zinn  into  the  capsule  of  the  lens 
(Testut).  I.  Posterior  lens  surface.  2. 
Its  equator.  3.  Zonula.  4-5.  The  an- 
terior and  posterior  bands.  6.  The  in- 
ter-fascicu-lar  spaces,  formerly  regarded 
as  the  canal  of  Petit. 


FIG.  40. — Fragment 
of  capsule  of  lens  near 
its  equator.  The  points 
reflected  backward  are 
the  attachments  of  the 
zone  of  Zinn  (Schoen). 


62  Structure  of  the  Lens 

pass  for  the  most  part  over  the  anterior  surface,  while  those 
which  arise  still  more  posteriorly  interlace  with  the  adjoining 
fibers  and  pass  partly  to  the  anterior  and  partly  to  the  pos- 
terior surface. 

This  interlacing  of  fibers  of  the  ligament  is  not  easy  to 
photograph,  the  fibers  seldom  lying  in  the  same  plane.  A 
drawing  by  Retzius  is  reproduced  here.  (Fig.  38.)  The 
physiological  importance  of  this  arrangement  is  evident, 
for  since  certain  special  portions  of  the  zonula  arise  from 
certain  portions  of  the  ciliary  process,  it  is  easy  to  under- 
stand how,  in  accommodation,  the  anterior  surface  of 
the  lens  may  become  relatively  more  convex  toward  the 
center,  while  the  posterior  portion  may  remain  unaffected. 
Another  schematic  view  of  the  zonula  is  given  in  Fig.  39 
from  Testut  and  also  a  part  of  the  capsule  near  the  equator 
of  the  lens  in  Fig.  40  from  Schoen. 

§  3.  Structure  of  the  Lens.— We  know  that  the  laminae 
which  make  up  the  lens  bend  over  its  edge  like  the  layers 
of  a  flattened  onion.  This  can  be  easily  seen  by  boiling  a 


FIG.    4i.-Boiled    lens    en-  FIG.  42.-Some  of  the  lay- 

larged    and   viewed   from  the  ers  of  the  lens  separated  from 

,  each  other  (Fr.  Noland). 


fresh  lens  (Fig.  41).  A  vertical  section  of  the  different 
layers  is  shown  in  Fig.  42  and  also  in  Fig.  43.  When  one 
of  these  layers  is  examined  more  minutely  we  find  that  it  is 
composed  of  so-called  ultimate  fibers,  each  one  fitting  ac- 


Structure  of  the  Lens 


curately  to  the  next.  (Fig.  44.)  As  these  ultimate  fibers 
pass  from  one  side  over  the  edge  of  the  lens  to  the  other 
side,  it  is  natural  to  conclude  that  their  elasticity  produces 
a  constant  tendency  in  each  fiber  to  stretch  out  more  nearly 
straight.  We  should  remember  also  that  the  superficial 
layers  are  by  no  means  as  dense  as  those  near  the  center. 
Indeed,  the  lens  may  be  considered  as  made  up  of  several 
strata,  each  composed  in  turn  of  a  number  of  ultimate  fibers, 
the  external  strata  being  decidedly  more  elastic,  and  having 
a  lower  index  of  refraction, 
than  those  which  lie  near 
the  center. 

The  foregoing  account  of 
the  structure  of  the  lens  ac- 
cords in  general  with  that 


FIG.   43.— Vertical  section  of    the 
lens  showing  its  laminae  (Merkel). 


FIG.  44. — Ultimate  fibers  of  the  lens 
(Merke!) 


which  is  found  in  most  of  the  text-books  or  digests  of 
physiological  optics.  But  such  a  description  has  one  great 
fault.  The  student  is  led  to  believe  that  when  parallel  rays 
fall  on  the  lens  they  all  converge  to  a  single  point,  and 
therefore  that  the  work  done  by  the  ciliary  muscle,  whatever 
that  may  be,  is  comparatively  simple.  In  reality,  however, 
such  is  not  the  case.  We  must,  therefore,  take  into  account 
these  imperfections  of  the  lens,  for,  as  we  shall  see,  they  in- 
dicate that  a  complicated  action  of  the  ciliary  muscle  is 
necessary  in  order  to  produce  a  more  or  less  perfect  image. 
Let  us  glance  at  a  few  of  these  peculiarities. 

First  are  the   irregularities  in    the   structure   of   the   an- 
terior  surface  of  the  capsule, —  the  chagrin  or  roughness 


64  Structure  of  the  Lens 

of  the  lens.     (Fig.  45.)     This  can  sometimes  be  seen  im- 
perfectly by  illuminating  the  lens  obliquely  and  examining 

the  spot  thus  lighted  through 
a  strong  loupe.  It  can  also  be 
made  visible  with  the  large 
horizontal  microscope,  but  the 
best  view  of  it  is  obtained  with 
the  corneal  microscope  of  Zeiss. 
When  seen  in  this  way,  the 
anterior  capsule  presents  a 
roughened,  irregular  aspect, 
not  unlike  the  paper  or  linen 

FIG.    45.  —  Appearance    under  ,  .   ,  ,  , 

magnification  of  the  anterior  sur-       which     COVCrS     the     Pasteboard 

face  (the  chagrin)  of  the  lens.  binding  of  certain  books.  This 

appearance  is  called  techni- 
cally the  "chagrin"  of  the  finish,  and  this  term  was 
used  also  to  describe  the  roughness  of  the  anterior  cap- 
sule by  Hess,  one  of  the  first  to  observe  it.  It  is  shown 
in  Fig.  45.  Any  one  who  will  take  the  trouble  neces- 
sary to  see,  in  this  way,  how  irregular  the  surface  of  the 
capsule  really  is,  is  inclined  to  regard  this  condition  as 
one  of  the  reasons  why  the  lens  does  not  always  give  a 
perfect  focus. 

A  second  imperfection  is  the  fine  radiating  opaque  lines 
which  quite  often  exist  near  the  periphery  of  the  lens. 
Every  practitioner  is  familiar  with  the  appearance  presented 
by  a  so-called  peripheral  cataract  in  the  early  stages.  Even 
with  an  undilated  pupil  it  is  then  easy  to  see  by  reflected  or 
transmitted  light  how  the  fine  lines  in  the  lens  streak  to- 
ward its  center.  Now  when  the  pupil  of  even  a  normal  eye 
is  moderately  dilated,  it  is  usually  possible  to  distinguish 
similar  lenticular  opacities,  especially  when  the  person  has 
passed  early  life. 

Third,  fine  opaque  lines,  irregularly  radiating  or  branch- 
ing, are  nearly  always  present  near  the  center  of  the  lens. 
These  apparently  correspond  to  the  raphes  produced  by  the 
joining  of  the  ultimate  fibers,  which,  after  starting  from 
such  a  line  near  one  surface  of  the  lens,  bend  over  its  edge 
to  form  this  line  on  the  other  surface.  Various  forms  of 


Structure  of  the  Lens  65 

these  lines  are  shown  in  Fig.  46.  They  were  studied  carefully 
by  Friedenberg,  and  can  be  seen  subjectively,  as  he  saw 
them,  by  simply  looking  through  a  minute  aperture.  The 
coarse  stenopaic  disc,  which  comes  in  the  usual  test  case, 


FIG.  46. — Five  samples  of  entoptic  spectra  as  seen  in  different  eyes,  these 
sectors  being  caused  by  the  radiating  lamellae  (Friedenberg). 

has  an  opening  too  large  to  make  this  observation  ac- 
curately. The  best  way  to  see  these  central  lines  entopti- 
cally  is  to  make  an  opening  with  a  very  fine  needle  in  a 
sheet  of  copper  which  has  been  rolled  out  to  the  thinness 
of  writing-paper.  Holding  this  with  the  hand — or  better, 
fixing  it  in  a  test  frame, — one  should  look  through  the 
small  hole  at  the  sky,  the  other  eye  meanwhile  being  covered. 
A  stenopaic  disc  of  this  kind  is  not  simply  an  instrument  to 
be  kept  in  the  test  case,  but  is  of  real  use.  For  when  the 


FIG.  47. — Two  entoptic  spectra  in  eyes  which  have  perfect  vision.  These 
were  seen  when  the  person  looked  at  the  sky  through  a  smooth  stenopaic  open- 
ing half  a  millimeter  in  diameter  (Hess). 

lines  and  the  spots  (which  will  be  mentioned  next)  are  espe- 
cially abundant  or  well  marked,  they  mean  a  difficulty  or 
impossibility  on  the  part  of  the  ciliary  muscle  to  produce 
a  perfect  focus,  and  therefore  a  more  unfavorable  prognosis 
of  any  existing  asthenopic  symptoms  must  be  given. 

Fourth,  one  or  more  small  spots  in  the  lens  are  usually 


66  Position  of  the  Lens 

seen  by  these  subjective  tests.  Ordinarily  one  is  quite  ap- 
parent, near  the  center  of  the  lens,  and  toward  it  the  radiat- 
ing lines  converge;  or  that  spot,  or  a  second,  or  possibly 
still  another,  barely  perceptible  and  more  or  less  eccentric, 
can  be  detected  with  patience  in  changing  the  position  of 
the  opening  before  the  eye  and  by  altering  the  degree  of 
the  accommodation. 

§  4.  Position  of  the  Lens. — We  are  apt  to  think  that 
the  axis  of  the  lens  coincides  with  the  optic  or  the  visual 
axis.  The  fact  is, that  the  lens  usually  faces  temporalward 
in  relation  to  the  visual  axis  and  usually  its  upper  edge  is 
tipped  forward,  or  it  is  otherwise  displaced  slightly  from  the 
position  which  it  is  usually  supposed  to  occupy. 

Now  this  tipping,  or  malposition  of  the  lens,  not  only 
produces  in  itself  a  slight  amount  of  astigmatism  (Fig.  48), 


FIG.  48. — The  focal  line  of  a  lens  which  is  placed  obliquely  (Tscherning). 

even  in  what  we  call  the  normal  eye,  but  clinical  experience 
indicates  that  in  many  instances  even  a  comparatively  small 
degree  of  malposition  of  the  lens  requires  such  traction  on 
the  ciliary  muscle  as  to  be  an  important  cause  of  what  we 
call  accommodative  asthenopia.  Since  making  it  a  habit  to 
measure  the  position  of  the  lens,  I  have  been  impressed  by 
the  number  of  cases  in  which  discomfort  and  symptoms  of 
asthenopia  were  associated  with  an  unusual  degree  of  this 
malposition.  It  is  difficult  to  say  why  astigmatism  pro- 
duced in  this  way  should  give  anymore  inconvenience  than 
an  ordinary  corneal  astigmatism,  although,  if  additional 
theories  were  desired  concerning  the  unequal  traction  thus 
made  directly  upon  the  fibers  of  the  ciliary  muscle,  it  would 


Position  of  the  Lens  67 

be  easy  to  build  one  up.  In  any  persistent  case  of  asthe- 
nopia,  no  examination  can  be  considered  complete  unless 
the  position  of  the  lens  with  regard  to  the  axis  of  vision  is 
also  taken  into  account.  We  must  therefore  examine  with 
considerable  care  the  methods  by  which  the  malposition  of 
the  lens  can  be  measured.  In  doing  so  let  us  glance  at  the 
simple  principle  involved,  as  illustrated  by  a  familiar  experi- 
ment, and  then  see  how  that  principle  is  worked  out  in  the 
instruments  ordinarily  used  to  measure  the  position  of  the 
lens. 

§  5.  How  can  we  Determine  the  Position  of  the  Lens? 
— Let  us  begin  by  observing  the  relative  positions  of  the  re- 
flections from  its  surfaces  (the  so-called  entoptic  images)  as 
compared  with  the  position  of  the  reflection  from  the  cornea. 
This  is  not  difficult.  The  reflections  from  the  cornea  and 
the  posterior  surface  of  the  lens  are  readily  seen  when,  in 
a  dark  room,  we  look  in  to  the  pupil  of  an  eye  upon  which 
the  light  from  a  candle  falls  obliquely.  (Fig.  49.)  We  see 


FIG.  49. — Relative  position  of  observed  eye  (A),  of  the  light  (C),  and  of  the 
observer's  eye  (£)  during  examination  of  the  entoptic  images  (Helmholtz). 

from  the  cornea  the  reflection  of  the  candle  flame  conspicu- 
ous and  comparatively  large,  and  from  the  posterior  surface 
of  the  lens  a  bright  point  much  smaller  than  the  first.  If 
we  then  look  with  much  care,  and  just  at  the  proper  angle, 
we  may  also  see  the  blurred  irregular  reflection  from  the 
anterior  surface  of  the  lens.  That,  though,  is  more  difficult 
to  recognize,  and  may  be  disregarded  for  the  present. 

The  point  of  interest  to  us  is  the  relative  position  of  the 
reflections  from  the  cornea  and  from  the  posterior  surface 
of  the  lens.  If,  in  a  dark  room,  the  observer  were  to  look 


68  Position  of  the  Lens 

straight  into  the  observed  eye  and  the  patient  were  to  look 
straight  back  at  the  observer,  the  visual  axes  of  both  coin- 
ciding, and  if  at  the  same  time  a  candle  were  held  slightly 
above  or  below  the  observer's  eye,  he  would  see  the  reflec- 
tions from  the  cornea  and  from  the  posterior  surface  of  the 
lens  in  the  same  vertical  line,  provided  the  axis  of  the  lens 
thus  looked  at  coincided  with  its  visual  axis. 

But  if  the  lens  in  the  observed  eye  faced  outward  or 
inward  a  little,  then  under  the  same  circumstances  the  ob- 
server would  see  these  two  reflections  arranged  as  in  Fig. 
50  a.  In  reality,  this  is  what  we  do  find  almost  invariably 

0OOO 

a  bed 

FIG.  50. 

FlG.  50  a. — Reflections  from  cornea  and  posterior  capsule  when  the  lens  is 
tipped  outward  (its  usual  position). 

(b) — Reflections  from  cornea  and  posterior  capsule  when  the  lens  is  in 
vertical  alignment. 

(<•) — Reflections  from  cornea  and  posterior  capsule  when  the  lens  is  tipped 
forward. 

(//) — Reflections  from  cornea  and  posterior  capsule  when  the  lens  is  in 
horizontal  alignment. 

when  this  examination  is  made  by  a  more  accurate  method 
presently  to  be  described.  Evidently  it  is  not  difficult  to 
ascertain  how  much  this  lateral  malposition  of  the  lens 
amounts  to,  for  if  the  lens  faces  temporalward,  as  usual, 
and  the  observed  eye  be  rotated  slowly  toward  the  median 
line,  a  point  is  found  at  which  the  reflections  from  the 
cornea  and  from  the  posterior  surface  of  the  lens  do  come 
into  the  same  vertical  line  as  in  Fig.  50  b,  and  the  number 
of  degrees  thus  traversed  horizontally  by  the  globe  is, 
of  course,  the  number  of  degrees  which  the  lens  faces 
outward. 

Or  it  may  be  that  the  lens  tips  vertically.     In  that  case, 


Position  of  the  Lens  69 

if  the  candle  were  held  directly  below  or  above  the  line 
connecting  the  foveae  of  the  observer's  and  of  the  observed 
eyes,  the  reflections  from  the  cornea  and  from  the  posterior 
surface  of  the  lens  might  still  be  in  the  same  vertical  line. 
But  if  the  candle  were  held  at  the  side,  the  two  reflections 
would  stand  as  in  Fig.  50  c.  Then,  if  the  observed  eye  were 
turned  slowly  up  or  down,  a  point  would  be  found  where 
the  two  reflections  would  stand  in  the  same  horizontal  line 
(Fig.  50^),  and  the  number  of  degrees  thus  traversed  up  or 
down  by  the  globe  would  be  the  number  of  degrees  of  ver- 
tical tipping  of  the  lens. 

This  examination  of  the  reflexes  with  the  candle  only, 
is  sufficient  to  illustrate  the  principle  involved.  If  ac- 
curacy is  required,  some  arrangement  must  be  devised 
for  holding  and  changing  at  will  the  position  of  the  light, 
and  for  viewing  the  reflections,  preferably  under  magni- 
fication, as  with  a  telescope.  A  movable  arc  must  also 
be  provided  on  which  to  measure  the  amount  which  the 
globe  is  rotated  in  any  given  direction.  Such  an  in- 
strument, very  complete  in  its  details,  has  been  designed  by 
Tscherning  and  called  by  him  an  ophthalmophacometer. 
But  before  examining  it  let  us  study  for  a  moment  a  modi- 
fication of  the  ophthalmometer  which  I  have  found  would 
serve  practically  the  same  purpose. 

§  6.  A  Modification  of  the  Javal  Ophthalmometer  for 
Estimating  the  Position  of  the  Lens. — As  the  instrument 
devised  by  Tscherning  is  rather  large  and  complicated,  and 
as  nearly  every  ophthalmologist  has  some  form  of  the 
Javal  ophthalmometer  which  contains  the  essential  parts 
of  the  former  instrument,  it  seemed  that  the  ophthalmometer 
might  be  so  modified  as  to  make  it  serve  the  purpose  of 
Tscherning's  ophthalmophacometer. 

This  is  done  as  follows: 

First.  The  inner  sheath  of  tubing  which  holds  the  prism 
in  the  barrel  of  the  instrument  is  removed,  and  arranged  so 
that  it  can  be  withdrawn  or  slipped  back  into  place  when 
desired,  as  a  cartridge  is  slipped  into  the  barrel  of  a  gun. 
A  slot  in  the  rim  of  the  outer  tube,  corresponding  to  a 
projecting  pin  on  the  cartridge  of  prisms,  holds  the  latter 


7O  Position  of  the  Lens 

exactly  in  position  (Fig.  51)  when  it  is  desired  to  use  the 
instrument  as  an  ophthalmometer,  or  when  the  prisma  are 
removed  the  instrument  becomes  a  simple  telescope. 


FIG.  51. — Arrangement  by  which  the  prisms  can  be  removed  from  the  Javal 
ophthalmometer,  thus  converting  the  instrument  into  a  telescope  for  determin- 
ing the  position  of  the  lens. 

Second.  A  small  electric  light  is  placed  six  or  eight  centi- 
meters below  the  center  of  the  arc,  turning  when  the  arc 
turns.  (Fig.  52.) 


FIG.  52. — Arrangement  of  the  Javal  ophthalmometer  when  converted  into 
a  telescope  with  a  light  below  and  a  movable  point  of  fixation  (a  glass  ball) 
above,  for  determining  the  position  of  the  lens. 

Third.  A  small  glass  ball  (a  hat  pin)  is  attached  horizon- 
tally to  the  top  of  one  of  the  mires,  the  conspicuous  head 


Position  of  the  Lens 


serving  as  the  point  of  fixation.  In  order  to  use  the  oph- 
thalmometer  as  an  ophthalmophacometer,  the  cartridge 
containing  the  prisms  is  removed,  the  lights  illuminating 
the  mires  are  ex- 
tinguished, the  sin- 
gle small  electric 
lamp  is  lighted,  the 
patient '  s  head 
placed  in  the  rest 
in  the  ordinary 
manner,  and  he  is 
directed  to  look  at 
the  fixation  point, 
which  is  placed  at 
first  just  above  the 
barrel  of  the  instru- 
ment. When  this  is 
done,  the  observer 
sees  the  reflection 
of  the  light  on  the 
cornea  (magnified 
by  the  telescope), 
and  below,  and  usu- 
ally toward  the 
inner  side,  there  is 
a  smaller  bright  re- 
flection from  the 
posterior  surface  of 
the  lens.  The  re- 
flection from  the 
anterior  surface  of 
the  lens  cannot  be 
seen  at  the 


FIG.  53.  FIG.  54. 

Relative  position  of  the  entoptic  images. 

FIG.  53. — When  the  observer  sights  along  the 
lighter  line  into  the  right  eye,  for  example,  of  the 
patient,  he  finds  the  entoptic  images  are  usually 
not  in  the  same  line. 

FIG.  54. — When,  however,  the  patient  has 
turned  his  eye  through  a  sufficient  arc  to  the  left, 
so  that  the  observer  looks  from  the  same  point 
along  the  heavy  line,  then  those  images  are  in  a 
vertical  line,  if  the  lens  is  not  otherwise  displaced. 


same 

time  because  of  the 
short  focal  distance 

of  this  telescope  (the  ophthalmometer).  The  position  of  the 
visual  axis  and  the  axis  of  the  lens  is  seen  in  Fig.  53 
and  also  the  relative  position  of  the  reflections  from  the 
cornea  and  from  the  lens.  If  the  small  glass  ball  be  now 


72  Position  of  the  Lens 

slid  along  the  arc  of  the  instrument,  a  point  is  reached  at 
which  the  reflections  are  in  the  same  vertical  line  (Fig.  54). 
The  angle  can  then  be  read  off  on  the  arc,  as  the  amount 
which  the  lens  turns  temporalward.  In  other  words,  this 
is  the  angle  alpha,  If  we  wish  to  measure  the  amount  which 
the  lens  tips  forward,  the  light  and  point  of  fixation  are 
placed  horizontally  by  turning  the  arc  vertically,  and  the 
measurement  made  as  before. 


FlG.  55. — Ophthalmophacometer  of  Tscherning.     The  arc  horizontal. 

When  the  ophthalmometer  is  thus  altered  into  an  oph- 
thalmophacoineter,  it  does  not  make  as  complete  an  instru- 
ment as  the  Ophthalmophacometer  of  Tscherning,  but  it  is 
simple,  it  is  convenient,  and  quite  sufficient  for  all  clinical 
purposes. 

§  7.  The  Ophthalmophacometer  of  Tscherning.—  If 
we  wish  to  ascertain  not  simply  which  way  the  lens  is  tipped, 
but  exactly  how  far  it  lies  behind  the  cornea,  and  especially 


Position  of  the  Lens 


73 


if  we  wish  to  observe  the  changes  in  its  two  surfaces  during 
accommodation,  it  is  necessary  to  use  the  ophthalmophaco- 
meter  of  Tscherning.  (B  264.)  (Figs.  55,  56.)  The  principle 
involved,  however,  is  just  as  simple  as, in  the  examination  of 
a  lens  with  a  candle,  or  with  the  modified  ophthalmometer 
just  described.  The  base  of  the  ophthalmophacometer  is 


FlG.  56.— Ophthalmophacometer  of  Tscherning.     The  arc  vertical. 

a  heavy  tripod,  supporting  a  pillar  of  iron  about  fifty  centi- 
meters high.  On  the  top,  a  telescope  is  firmly  fixed,  which 
has  a  focal  distance  of  about  eighty-five  centimeters.  The 
telescope  as  described  by  Tscherning  has  a  magnifying 
power  of  about  twelve  diameters,  but  it  gives  an  inverted 
image.  This  inversion  is  very  confusing,  however,  and 
requires  the  observer  to  make  a  mental  transposition  of 


74  Position  of  the  Lens 

the  images  in  order  to  understand  their  relative  positions. 
Consequently  after  some  trials  I  adjusted  to  the  tube  an 
erecting  eyepiece,  which  decidedly  simplifies  the  examina- 
tions1 (Fig.  57).  The  axis  of  the  telescope  forms  one  of  the 


FIG.  57. — Erecting  Eye  piece. 

radii  of  a  brass  arc,  whose  concavity  is  directed  forward,  the 
arc  having  a  radius  of  about  85  centimeters.  The  object  of 
this  long  radius  is  to  place  the  telescope  so  far  from  the  ob- 
served eye  that  it  is  possible  to  focus  at  the  same  time 
the  reflections  from  the  cornea,  from  the  posterior,  and  also 
the  one  from  the  anterior  surface  of  the  lens.  This  arc  is 
so  attached  as  to  revolve  about  the  axis  of  the  telescope, 
and  the  degrees  are  numbered  from  zero  in  the  center  to 
about  thirty  on  either  side.  On  this  brass  arc  three 
"cursors"  or  "carriers"  are  fitted  to  move  from  side  to 
side  as  desired. 

These  are: 

Carrier  A,  which  bears  one  electric  light. 

Carrier  B,  which  bears  an  upright  bar  on  which  there  are 
two  lights. 

Carrier  C,  which  bears  an  upright  bar  on  which  there  is  a 
small  ball  or  bright  button  at  which  the  patient  is  to  look. 
If  any  one  of  the  different  lamps  be  lighted  (that  on  carrier 
A,  for  example)  and  brought  near  the  center  of  the  arc,  we 
see  through  the  telescope  the  three  reflections  already  de- 
scribed. The  one  from  the  surface  of  the  cornea  is  well 
defined  and  the  brightest  of  the  three;  the  reflection  from 
the  anterior  surface  of  the  lens  is  blurred,  dull,  and  can  be 
distinguished  with  difficulty;  and,  finally,  the  reflection  from 
the  posterior  surface  of  the  lens  is  small,  bright,  and  easily 
seen. 

Now  while  the  study  of  these  reflections  is  of  interest  in 
different  ways,  our  object  at  present  is  simply  to  determine 
the  number  of  degrees  which  the  lens  faces  temporalward, 

1  Transactions  of  the  Medical  Society  of  the  State  of  New  York,  I  goo. 


Position  of  the  Lens 


75 


or  upward  or  downward.  For  that  purpose  all  that  we  actu- 
ally need  is  carrier  A  in  the  center  and  carrier  C,  the  fixa- 
tion point.  With  these  alone,  this  instrument  of  Tscherning 
is  practically  the  same  as  the  modification  of  the  ordinary 
Javal  ophthalmometer  just  described.  But  still  greater  ac- 
curacy is  possible  in  determining  the  position  of  the  lens. 
With  this  we  can  also  measure  the  changes  which  its  sur- 
faces undergo  during  the  act  of  accommodation,  if  instead  of 
one  source  of  light,  such  as  we  have  on  carrier  A,  we  use 
two  lights,  as  on  carrier  B.  In  order  to  do  this  we  extin- 
guish the  one  light  of  carrier  A,  push  that  aside,  and  bring- 
ing the  two  lights  of  carrier  B  in  the  line  of  the  axis  of  the 
telescope  we  obtain  six  reflections,  usually  scattered  over 
the  field  as  in  Fig.  58.  If  the  axis  of  the  lens  can  be  made 


FlG.  58. — Six  reflections  from  two  lights  when  the  lens  is  not  in  alignment 
(Tscherning). 

to  coincide  with  the  axis  of  the  instrument,  then,  by  regu- 
lating the  distance  between  the  two  lamps  on  carrier  B,  the 
reflections  of  one  of  these  lamps,  which  come  from  the 
anterior  and  posterior  surfaces  of  the  lens,  can  be  superim- 
posed on  the  reflections  of  the  other  lamp  from  the  posterior 
and  anterior  surfaces  respectively.  The  result  is  an  appear- 
ance like  Fig.  59.  But  the  interesting  fact  is,  that  this  is 
by  no  means  easy  to  accomplish,  because  of  the  frequent 
abnormalities  of  the  lens.  Usually  the  displacement  of  the 


76 


Position  of  the  Lens 


reflections  is  similar  to  that  seen  in  Fig.  58.  Or,  if  these 
reflections  are  watched  during  the  act  of  accommodation, 
the  changes  in  their  relative  positions  are  complicated  and 
not  always  possible  to  explain.  A  consideration  of  those 
changes  would  lead  us  too  far  from  our  object  at  present, 


FIG.  59. — Reflections  from  same  two  lights  when  the  lens  is  in 
(Tscherning). 

which  is  to  ascertain  simply  the  malposition  of  the  lens  and 
just  how  much  it  is  tipped  on  the  vertical  or  the  horizontal, 
or  on  an  oblique  axis. 

§  8.  Other  Refracting  Media  which  may  Influence  the 
Ciliary  Muscle. — Evidently  these  are  the  cornea  and  the 
vitreous.  Let  us  consider  them  in  order. 

(A)  The  Cornea  is  too  often  considered  clinically  as  a 
membrane  whose  curvature  is  equal  in  all  directions.  Or 
when  we  measure  that  curvature  -with  the  ophthalmometer 
and  find  an  astigmatism,  we  are  apt  to  infer  that  an  irregu- 
larity of  the  same  degree  extends  to  every  part  of  the 
cornea.  Now  the  fact,  frequently  forgotten,  is  that  with 
the  ophthalmometer  we  really  measure  only  the  curvature 
of  a  very  small  area  near  the  center  of  the  pupil,  and  al- 
though the  curvature  of  the  other  parts  of  the  cornea 
usually  approaches  that  of  the  center,  it  is  seldom  or  never 
the  same.  Sometimes  it  is  quite  different.  In  fact,  an  eye 
whose  vision  is  =  f,  and  whose  cornea  shows  only  0.5  D  of 


Other  Causes  of  "  Resistance  " 


77 


astigmatism  in  its  central  portion,  will  often  show  twice  or 
three  times  as  much  if  we  make  the  same  measurements  far- 
ther from  the  center.  The  works  on  physiological  optics 
nearly  all  refer  to  this  ir- 
regularity of  the  cornea, 
and  its  consequent  ten- 
dency to  produce  not  the 
perfect  focus  which  we  see 
figured  in  the  text-books, 
but  instead,  an  irregular 
crossing  of  rays  with  con- 
sequent circles  of  diffusion. 
(Fig.  60.) 

(B)  The    Vitreous  is   sel- 
dom  clear.      Nearly  every- 
one who   has  passed  early 
life    can     see    the    muscae 
volitantes  in  his  own  eyes 
when  he   looks  •  through  a 
small  stenopaic  opening  at 
a  bright  light,  though  it  is 

probable   that    these  have  usually  comparatively  little  in- 
fluence upon  the  exactness  of  the  focus  formed. 

(C)  Combined  Effect  of  Imperfections  of  the  Media. — The 
various  imperfections  in  the  refractive  media  combine  to 
produce  an  imperfect  focus,  especially  when  the  pupil  is 
rather  large.     This  important  clinical  fact  is  often  ignored. 
It  is  difficult  even  for  the  advanced  student  to  rid  his  mind 
of  those  elementary  diagrams  which  show  all  the  rays  com- 
ing together  just  at  a  single  point.     It  would  be  better  for 
him  to  recall  another  diagram  (Fig.  61),  even  though  this 
shows  the  rather  unusual  course  which  the  rays  take,  espe- 
cially with  a  wide  pupil.     With  such  a  condition  as  is  rep- 
resented in  this  diagram,  evidently  it  is  impossible  to  obtain 
a  perfect  focus.     There  may  be  one  or  two  points  where 
that  is  fairly  good,  but  clear  vision  is  obtained  only  by  an 
effort  of  the  ciliary  muscle  to  keep  the  lens  adjusted  with 
reference  to  a  certain  point — say  o  or  u — as  may  be  required 
for  the  far  or  near  point.     Above  all,  the  sphincter  of  the 


FIG.  60. — Form  of  the  focus  which 
is  made  by  a  cornea  with  comparatively 
slight  degree  of  unequal  curvature  near 
its  center  (Gullstrand). 


78  Other  Causes  of  "  Resistance  " 

iris  must  be  held  disproportionately  tense  in  order  to  keep 
the  pupil  small.  The  part  which  the  iris  and  the  ciliary 
muscle  play  in  overcoming  these  circles  of  diffusion  is  fa- 
miliar to  us  all  in  the  effect  of  atropin.  It  is  a  common 


FIG.  61. — Schematic  representation  of  the  visual  focus,  o  and  u  being  special 
focal  points  (Hess). 

experience  to  find  a  young  person  who  has  vision  of  %  and 
little  or  no  error  in  the  refraction  apparent  by  simple  tests; 
we  apply  atropin,  the  vision  falls  to  f  or  even  -fy>  and  we 
cannot  improve  it  with  any  glass  as  long  as  the  pupil  is 
dilated. 


FIG.  62. — Diagram  showing  the  manner  in  which  the  chromatic  aberration 
of  a  human  lens  corrects  itself.  In  this  it  will  be  seen  how  the  red  ray,  rly  ra, 
from  one  portion  of  the  periphery,  coincides  with  the  violet  ray,  vlt  z>8,  from 
another  portion  of  the  periphery  (Hess). 

Perhaps  we  might  wonder  why  it  is  that  such  persons 
do  not  complain  of  chromatic  aberration  and  all  sorts  of 
similar  difficulties.  This  and  like  questions  have  been 
asked  and  answered  by  Helmholtz,  Volkmann,  Hering, 
and  others.  The  reason  we  do  not  notice  the  fringe  of 
colors,  for  example,  has  been  satisfactorily  explained  by  the 
overlapping  of  the  spectra.  Thus,  in  Fig.  62,  as  the  ray 
A  A,  enters  the  eye,  when  unequally  refracted  by  any  of  the 
media  the  violet  portion  is  bent  toward  v±  and  the  red  por- 


Other  Causes  of  "  Resistance  "  79 

tion  toward  r1.  The  ray  A  Alt  is  also  spread  out  into  vz 
and  rz,  but  as  these  circles  of  diffusion  constantly  overlap, 
the  red  ray  falls  upon  the  violet  one  often  enough  to  correct 
the  impression  of  chromatic  aberration. 

In  a  similar  manner,  Helmholtz  has  shown  that  when  the 
retinal  image  is  imperfect  a  portion  of  the  impression  is 
suppressed,  as  it  were,  following  the  law  of  contrasts,  and  in 
that  way  the  difficulty  is  obviated,  at  least  in  part.  For 
our  purpose  it  is  unnecessary,  even  if  it  were  possible,  to 
enter  into  details  concerning  the  correction  of  chromatic 
aberration  or  other  imperfections  of  the  retinal  image. 
Suffice  it  to  say  that  they  exist  more  or  less  markedly  in 
practically  every  eye. 

§  9.  Of  what  Clinical  Importance  are  Abnormal  Posi- 
tions of  the  Lens  or  Imperfections  of  the  Refractive  Me- 
dia ? — The  question  may  perhaps  be  asked,  what  is  there  in 
a  study  of  the  muscles  of  the  eye  to  warrant  so  much  atten- 
tion to  the  structure  and  position  of  the  lens,  to  instruments 
for  measuring  it,  and  to  imperfections  of  the  refractive  me- 
dia? At  first  glance  these  topics  may  seem  quite  foreign  to 
the  ocular  muscles.  But  in  the  clinical  part  of  our  study  we 
must  keep  constantly  in  mind  the  fundamental  fact  that  the 
whole  aim  of  the  ciliary  muscle,  and  secondarily  of  the  other 
muscles,  is  to  form  on  the  retina  as  clear  a  focus  as  possible. 
Anything  which  interferes  with  that  is  an  element  in  the 
resistance  offered  for  the  intraocular  muscles  to  overcome. 
For  example,  we  have  long  ago  recognized  the  fact  that 
even  a  slight  amount  of  corneal  astigmatism  does  in  certain 
individuals  produce  very  decided  asthenopic  symptoms, 
those  symptoms  disappearing  when  the  imperfection  of  the 
cornea  is  corrected  by  a  suitable  glass.  But  a  malposition 
of  the  lens  can  also  produce  an  astigmatic  focus.  Indeed, 
that  malposition  may  be  of  such  a  kind  as  to  make  its 
optical  correction  impossible.  Evidently  such  a  condition 
must  influence  the  prognosis  very  decidedly,  and  the  de- 
sirability is  apparent  of  recognizing  not  only  the  condi- 
tion, but  its  degree.  In  like  manner  imperfect  foci  may  be 
produced  by  irregularities  in  the  density  of  different  por- 
tions of  the  lens,  or  by  the  unequal  curvature  of  its  surfaces, 


8o  The  Accessory  Muscles 

especially  in  the  act  of  accommodation,  or  by  relatively 
dense  spots  either  in  the  lens  or  the  vitreous. 

It  is  only  necessary  to  recall  the  fact  that  any  one  of  these 
imperfections  constitutes  a  factor  in  what  may  be  called  the 
resistance  to  normal  muscular  action,  and  we  appreciate 
immediately  that  each  of  these  may  be,  and  often  is  of  de- 
cided clinical  importance. 

In  this  connection  it  should  also  be  remembered  that  any 
irregularities  in  the  curvature  of  the  cornea  or  of  the  lens, 
or  opacities  in  the  refractive  media  which  ordinarily  do  not 
impede  vision  because  of  the  contraction  of  the  pupil,  do 
become  more  noticeable  after  a  cycloplegic  has  been  used. 
All  of  this  emphasizes  the  fact  that  when  we  use  atropin 
the  condition  of  the  refraction  then  obtained  does  not  rep- 
resent the  actual  refraction  of  the  normal  eye. 

§  10.  Accessory  Muscles  of  Accommodation. — Writers 
have  apparently  overlooked  to  a  great  extent  the  role  played 
by  the  accessory  muscles  of  accommodation.  It  is  worth 
while,  therefore,  to  recall  them,  not  only  for  completeness, 
but  because  of  their  undoubted  clinical  importance  in  con- 
nection with  certain  forms  of  ocular  headaches. 

The  first  of  these  accessory  muscles  is  the  corrugator 
supercilii.  According  to  Gray,  this  is  a  small  pyramidal 
muscle  placed  near  the  median  line,  beneath  the  occipito- 
frontalis  and  the  orbicularis  palpebrarum.  It  arises  from 
the  inner  extremity  of  the  superciliary  ridge,  its  fibers  pass- 
ing outward,  to  be  inserted  into  the  under  surface  of  the 
orbicularis  palpebrarum  opposite  the  middle  of  the  orbital 
arch.  There  are  decided  variations,  however,  in  this  muscle, 
as  in  others  of  the  group,  and  also  in  the  distribution  of  the 
fascia  near  the  center  of  the  forehead. 

These  variations  are  appreciated  when  we  compare  the 
drawings  given  by  Gray,  Fig.  63,  and  by  Henle,  Fig.  64, 
and  become  very  evident  after  even  a  few  dissections. 

The  second  group  of  fibers  to  be  noted  includes  those  of 
the  pyramidalis  nasi.  These  arise  from  the  edge  of  the 
orbicularis,  and  with  those  on  the  opposite  side  pass  down- 
ward  covering  the  upper  part  of  the  nose.  Usually  this 
muscle  is  very  imperfectly  developed. 


The  Accessory  Muscles 


81 


FIG.  63.— Muscles  of  the  face  and  head  (Gray). 


as. 


I.P  L. 


E.PL. 


FIG.  64. — Muscles  of  the  upper  portion  of  the  face  (Henle). 


82 


The  Accessory  Muscles 


The  third  and  by  far  the  most  important  accessory  muscle 
of  accommodation  is  the  occipito-frontalis.  According  to 
Gray  and  Henle  the  occipital  portion  of  this  muscle  arises 

from  the  outer  half  or 
two-thirds  of  the  supe- 
rior occipital  ridge  and 
from  the  mastoid  por- 
tion of  the  temporal. 
The  few  fleshy  fibers 
forming  this  portion 
soon  become  tendinous 
and  pass  upward  as 
an  aponeurosis  which 
covers  the  whole  pos- 
terior and  upper  por- 
tion of  the  skull.  (Fig. 
65.)  As  the  anterior 
continuation  of  that 
aponeurosis,  or  skull- 
cap, we  have  the  frontal 
fleshy  portion  of  this 
muscle.  It  is  quadri- 


FIG.  65. — Schematic  representation  of  the 
occipito-frontalis  muscle  (Testut).  i.  Frontal 
portion.  2.  Occipital  portion.  3.  Aponeu- 
rosis connecting  these  two  portions. 


lateral  in  form,  and  as 
the  fibers  come  forward 

a  few  pass  downwards  to  blend  with  the  fibers  of  the 
orbicularis,-with  the  corrugator,  and  other  muscles  at  the 
root  of  the  nose. 

The  contraction  of  the  anterior  fibers  of  the  occipito- 
frontalis  causes,  as  we  know,  the  well-known  horizontal 
wrinkling  of  the  forehead  giving  the  expression  of  surprise, 
fear,  etc.,  as  has  been  shown  by  Darwin.  In  certain  forms 
of  muscle  imbalance,  especially  when  the  vertical  muscles 
are  affected,  the  eyebrows  are  raised  and  these  horizontal 
wrinkles  become  particularly  prominent.  Reference  will  be 
made  to  this  later.  The  foregoing  description,  with  more 
or  less  elaboration,  we  find  in  the  classical  works  on  anatomy. 
The  origin  and  attachments  of  this  muscle  are,  however, 
worthy  of  more  exact  study.  Apparently  there  is  no  exact 
description  of  the  connection  between  its  posterior  portion 


The  Accessory  Muscles  83 

and  the  trapezius  muscle.  It  is  not  difficult,  however,  to 
see  that  these  two  muscles  are  quite  intimately  joined. 
For,  in  making  dissections  of  the  tissue  over  the  superior 
curved  line  of  the  occipital  bone,  if  the  superficial  fascia  be 
first  removed,  it  will  be  observed  that  the  deeper  fascia, 
which  is  quite  adherent  to  the  posterior  portion  of  the 
occipito-frontalis,  is  also  adherent  to  the  trapezius.  The 
outline  drawing  (Fig.  66)  illustrates  imperfectly  the  con- 
nection between  these  two  muscles.  As  the  fibers  of  the 
trapezius  muscle  curve  downwards  over  the  shoulders,  it  is 
evident  that  if  in  forcible  accommodation  the  individual 
scowls  and  wrinkles  the  skin  of  the  forehead,  that  tends  to 


FlG.  66. — Diagram  showing  the  manner  in  which  the  fibers  of  the  occipito- 
frontalis  continue  into  those  of  the  trapezius.  A  part  of  each  muscle  is  lifted 
by  each  pair  of  forceps. 

draw  also  on  the  posterior  fibers  of  the  occipito-frontalis, 
and  through  them,  the  traction  extends  to  the  trapezius 
as  has  been  already  mentioned.  Later,  reference  will  be 
made  to  this  fact  in  connection  with  headaches,  pain  in  the 
occiput,  or  back  of  the  neck. 

Fourth.  The  orbicularis  palpebrarum.  This  is  not  usu- 
ally classed  with  the  accessory  muscles  of  accommodation, 
and  yet,  the  more  one  studies  the  arrangement  of  its  fibers, 
and  also  those  contractions  of  the  lids  which  come  with  ex- 
cessive effort  at  accommodation,  the  more  does  it  appear, 
from  a  clinical  standpoint,  to  belong  to  this  group. 

A  word  should  be  added  concerning  it,  not  only  for  the 
reason  above  stated,  but  also  because  of  its  antagonistic 
action  to  the  levator  palpebrae.  The  description  of  it  in 


84  The  Accessory  Muscles 

the  different  text-books  varies  as  much  as  the  muscle  itself, 
and  these  differences  are  not  slight,  as  will  be  found  by  any 
one  who  will  take  the  trouble  to  make  a  few  dissections. 
Most  of  the  English  text-books  describe  it  as  practically 
one  sphincter  muscle,  the  fibers  of  which  pass  around  the 
palpebral  fissure  above  and  below  from  the  palpebral  liga- 
ment. This  arrangement  is  simple  and  probably  exists  in 
some  subjects.  More  extended  descriptions  of  it  are  given 
by  Henle,  Merkel,  and  others.  The  special  point  of  interest 
to  the  ophthalmologist  is  that  this  is  not  a  single  muscle, 
but  practically  is  made  up  of  three.  The  first  part  covers 
the  lid  itself  above  and  below.  The  second  part  is  almost  a 
distinct  oval  concentric  with  the  first,  while  the  third  tends 
to  form  another  oval,  just  outside  of  the  second,  this  last 
one  being  composed  of  irregular  fibers  more  or  less  devel- 
oped, which  extend  over  the  face  (Henle)  or  upward  to 
join  the  occipito-frontalis,  blending  with  that  muscle.  Thus 
we  have  three  muscles  apparently  with  different  actions. 
The  first  portion  contracts  in  the  act  of  winking.  The 
second  acts  with  the  first  in  winking  or  in  the  frowning 
which  accompanies  accommodation.  The  fibers  of  the  third 
group  are  only  brought  into  action  when  a  strong  effort  at 
accommodation  is  long  continued,  or,  of  course,  when  the 
lids  are  forcibly  closed. 


CHAPTER  III. 
THE   NERVE   SUPPLY   OF   THE    MUSCLES. 

§  i.  General  Considerations  and  Macroscopic  Anat- 
omy.— The  importance  of  this  aspect  of  our  subject  is  self- 
evident.  Some  writers  even  go  so  far  as  to  say  that  all 
myology  resolves  itself  into  neurology,  and  while,  in  view 
of  the  anatomical  differences  in  the  muscles  themselves  and 
in  their  insertions,  that  statement  is  more  epigrammatic 
than  true,  it  nevertheless  expresses  a  popular  opinion. 

It  is  possible  to  refer  to  the  subject  only  briefly  here, 
nor  is  it  desirable  to  do  more,  as  the  macroscopic  anatomy 
especially  is  known  to  every  student,  or  can  be  referred  to 
in  familiar  text-books.  It  is  well  in  approaching  this  part 
of  our  study  to  recall  the  nomenclature,  formerly  so  confus- 
ing, as  Barker  points  out  (B  193),  but  which  has  been  modi- 
fied of  late  years.  We  must  remember  that  the  brain  which 
is  under  examination  is  supposed  to  be  held  relatively  in 
position  and  immediately  in  front  of  the  student.  We  can 
then  understand  how  that  which  is  posterior  or  spinalward 
is  also  proximal,  while  that  which  is  cerebralward  is  also 
distalward.  Transverse  means  on  the  horizontal  plane. 
A  frontal  section  is  that  which  is  at  right  angles  to  the  long 
axis  of  the  medulla.  Median  is  the  central  vertical  plane 
from  before  backward,  while  sagittal  is  also  in  a  vertical 
plane  from  before  backward,  but  not  necessarily  in  the 
median  line.  Although  these  definitions  may  seem  ele- 
mentary, they  are  necessary  in  view  of  the  too-prevalent 
confusion  concerning  them. 

The  nerves  which  supply  the  ocular  muscles  are  the  third, 
fourth,  the  ophthalmic  branch  of  the  fifth,  and  the  sixth. 
When  we  turn  to  Gray,  or  any  of  the  standard  works  on 
anatomy,  we  find  these  arranged  in  the  order  familiar  to 

85 


86          Nerve  Supply — Macroscopic  Anatomy 

every  student.  In  reality,  the  parts  as  a  rule  are  more  dis- 
torted than  usually  represented,  and  more  like  the  illus- 
tration given  by  Barker.  (Fig.  67.)  It  is  not  easy  to 
remove  the  brain  so  as  to  obtain  a  good  view  of  all  the 
nerves,  for  the  reason  that  the  fourth  is  so  delicate  and  its 
origin  so  loosely  attached  that  it  is  frequently  torn  away  in 


FIG.  67. — Base  of  the  brain  (Barker). 

drawing  the  brain  forward.  When,  however,  a  satisfactory 
specimen  is  obtained,  we  see  that  the  four  nerves  which  are 
of  interest  in  this  connection  all  leave  the  brain  from  the 
pons  Varolii  or  immediately  adjacent  to  it.  This  part,  then, 
evidently  requires  close  inspection. 

Figure  68  gives  as  good  a  view,  perhaps,  as  any,  of  the 
anterior  surface  of  the  pons  with  its  relation  to  the  medulla 
oblongata,  while  the  posterior  view  (Fig.  69)  shows  the 


Nerve  Supply — Macroscopic  Anatomy         87 

fourth  ventricle  as  it  appears  when  its  roof  is  parted  in  the 
middle,  as  if  pushed  to  either  side.  Fig.  70  shows  the  ar- 
rangement of  alternate  trans- 
verse and  longitudinal  fibers  as 
they  appear  in  a  section  through 
the  median  plane.  It  must  be  re- 
membered, however,  that  these 


Middle 
'peduncle  of 
cerebellum. 


Restiform  body. 
Clara. 
Cuneate  tuberde. 


FIG.  68. — Medulla  oblongata  and  pons,          FIG.   69.— Medulla  oblongata 
anterior  view  (Gray).  and  pons,  posterior  view  (Gray). 


FIG.   70. — Median  section  of  medulla  oblongata  and  pons.     Diagrammatic 
(Gray). 

drawings   are   in    some  respects  hardly  more  than  rough 
diagrams.     They    indicate  the  direction  of  the  fibers  and 


88         Nerve  Supply — Macroscopic  Anatomy 

give  their  names,  but  do  not  represent  what  the  student 
of  anatomy  really  sees.  A  better  idea  of  a  median  section 
through  this  region  is  given  by  Figure  71.  This  shows  the 
relative  size  and  the  position  of  the  fourth  ventricle,  with 
the  structures  in  that  vicinity. 

The  view  of  the  base  of  the  skull  (Fig.   72)  shows  the 
foramina  through  which  the  nerves  make  their  exit. 


f 


FIG.  71. — Sagittal  section  through  the  pons.  In  making  this,  the  aqueduct 
of  Sylvius  was  opened  at  one  small  point,  as  shown  by  the  dot  nearly  in  the 
center  of  the  figure  between  the  third  and  fourth  ventricles. 

Having  recalled  these  preliminary  facts  of  macroscopical 
anatomy,  and  the  general  arrangement  of  the  structures  in 
the  region  of  the  fourth  ventricle,  we  are  better  prepared  to 
study  the  deep  origin  of  the  nerves  which  supply  the  ocular 
muscles.  It  is  instructive  also  to  glance  at  the  grouping  of 
the  cells  in  the  medulla  and  pons  before  studying  the  details 
of  each  group  separately.  Their  arrangement  is  shown  in 
the  admirable  diagram  first  given  by  Edinger  (B  144)  and 
reproduced  here.  (Fig.  73.)  The  most  important  struc- 
tures in  the  pontine  system  lie  almost  immediately  below 
the  floor  of  the  fourth  ventricle.  An  idea  of  these  can  be 


Nerve  Supply — Microscopic  Anatomy          89 

obtained  by  making  a  median  section  and  then  carefully 
dissecting  off  the  thin  layer  of  fibers  which  constitutes  the 
floor  of  that  ventricle.  Unfortunately,  this  is  by  no  means 
an  easy  task,  as  the  interlacing  of  the  fibers  makes  their 


Opficus 


Oculomoforius 
Sinus  cavernosus  .. 
Abclucens 

Car-otis 
TrocMearis 
Trigeminus 


FIG.   72. — View  of  part  of  the  base  of  the  skull,    showing    the   foramina 
through  which  the  nerves  make  their  exit  (Bernheimer). 

•si 


FIG.  73. — Diagrammatic  representation  of  the  origins  of  the  cranial  nerves 
in  the  pons  and  in  the  medulla  (Edinger). 

separation,  especially  at  certain  points,  almost  impossible. 
The  best  views  of  the  arrangement  are  obtained  by  serial 
sections  perpendicular  to  the  axis  of  the  pons  and  also  in 
various  vertical  planes.  It  is  from  such  dissections  and  thin 
sections,  especially  of  the  foetus,  that  we  learn  the  direc- 
tion of  the  fibers  in  this  important  portion  of  the  brain. 
A  general  view  of  this  arrangement  is  seen  in  Fig.  74. 


90      The  Third  Nerve — Microscopic  Anatomy 

§  2.  Third  Nerve  (Motor  Oculi). —  The  origin  of  this 
nerve  presents  three  parts  for  examination.  These  are: 
(A)  The  nucleus  itself.  (B)  A  group  of  cells  in  the  gyrus 
angularis.  (C)  Fibers  which  connect  these  two  portions. 
Let  us  consider  them  in  order. 

(A)  The  Nucleus  in  the  Pans. — Most  of  the  fibers  com- 
posing this  important  nerve  arise  from  certain  groups  of 
cells  which,  together,  we  call  the  nucleus.  This  lies  just 
beneath  the  aqueduct  of  Sylvius,  near  its  posterior  or 
proximal  end,  part  of  the  nucleus  lying  on  one  side  of  the 
median  plane  and  part  on  the  other.  If  the  entire  nucleus 
were  dissected  out,  it  might  be  described  as  roughly  egg 
shaped,  five  to  six  millimeters  long  from  before  backwards, 
and  broader  posteriorly  than  anteriorly.  Closer  inspection 
shows  that  it  is  made  up  of  two  equal  irregular  masses 
which  are  united  in  the  median  plane.  Each  part  is  slightly 
concave  externally,  having  rather  sharp  converging  edges 
internally  and  downward,  and  divergent  smaller  ends  ex- 
ternally and  upwards. 

Let  us  consider  next  the  microscopic  structure  of  this 
nucleus  and  the  arrangement  of  the  different  groups  of  cells 
of  which  it  is  composed.  This  arrangement  is  ascertained 
by  the  study  of  serial  sections,  the  observations  of  various  his- 
tologists  being  virtually  in  accord,  no  matter  which  method 
of  staining  is  adopted.  One  of  the  most  complete  of  these 
descriptions  is  given  by  Bernheimer  (B  195).  Copies  are 
here  shown  of  three  frontal  sections,  but  it  is  practically  im- 
possible to  reproduce  the  different  groups  of  cells  exactly 
even  with  the  contrast  of  different  colors.  Of  these  sections 
Fig-  75  is  near  the  posterior  end  of  the  nucleus,  Fig.  76 
near  the  center,  and  Fig.  77  nearer  the  anterior  end. 

When  we  study  these  sections  we  find  that  all  the  cells 
arrange  themselves  into  about  five  principal  groups, 

(a)  On  each  side,  near  the  lateral  portion  of  the  nucleus, 
is  a  considerable  group  which  we  will  call  a  and  a'.  These 
are  the  so-called  large  lateral  cells.  It  is  probable  that  this 
group  may  itself  be  subdivided  into  smaller  groups  of  cells, 
but  some  sections  indicate  that  the  groups  merge  into  each 
other.  The  principal  part  of  this  group  is  made  up  of 


Nerve  Supply — Microscopic  Anatomy 


(NUCLEUS 

\      N.  OcULOMOTORIf 


Nucleus 

colliculi  inferiorisl 

Radix 

N.  trochlearisj 


RADIX 

N.  trigemini 

NUCLEUS 
MOTORIUS 
N.  TRIGEMINIJ,. 

NUCLEUS  i 

N.  TRIGEMINIf 

(SENS.) 

Corpus         V 
restiforme/ 


Corpus 
restiforme/ 


Mucleus  j 
funiculi) 
cuneati  ' 


nuclei 

colliculi 

inferioris 


(NUCLEUS 

I    N.  TROCHLEARIS- 


/Ventriculus 
•\     quartus 
'Corpus 
.1    restiforme 
/RADIX 
V    N.  FACIALIS 

"(NUCLEUS 
\    N.  ABDUCENTIS 


(Nucleus 
-\    olivaris; 
I     inferior 

•(Nucleus 

N.  hypoglossl 


FIG.  74. — General  view  of  the  pontine  system  and  of  the  structures  lying 
beneath  the  floor  of  the  fourth  ventricle  (Sabin's  model  of  this  part  of  the 
brain). 


The  Third  Nerve — Microscopic  Anatomy      97 

multipolar  cells  of  considerable  size,  about  forty  micro- 
millimeters  in  diameter,  which,  when  properly  stained, 
show  a  central  granular  nucleus.  From  these  cells  ultimate 
nerve  fibers  pass  basalwards,  joining  with  other  fibers  to 
form  the  nerve  trunk  on  either  side.  Von  Gudden  (B  112) 
considers  it  certain  that  at  least  a  partial  crossing  of  the 
fibers  from  one  side  to  the  other  does  occur. 


FIG.  75. —  Frontal  section   near  the   posterior   end   of    the 
nucleus  of  the  motor  oculi  (Bernheimer). 

(b)  On  each  side,  above  and  near  the  median  line,  is  a 
small  group  of  small  cells  (b  and  b').  As  the  upper  end  of 
each  one  of  the  main  groups  of  large  cells  (a  and  a')  bends 
away  from  the  median  plane,  it  leaves  a  space  which  is 
occupied  above  by  this  so-called  supplementary  group. 
The  cells  which  compose  this  group  are  perceptibly  smaller 
than  those  in  the  main  lateral  group,  and  give  to  the  stain 
a  lighter  color.  This  large  group  of  large  cells  (a  and  a'), 
together  with  the  small  group  of  small  cells  (b  and  b')  situ- 
ated on  each  side  of  the  median  line,  constitute  what  may 


92      The  Third  Nerve — Microscopic  Anatomy 

a 


FIG.  76. — The  same  near  the  central  portion  (Bernheimer). 


FIG.  77. — The  same  near  the  anterior  portion  (Perlia). 


The  Third  Nerve — Microscopic  Anatomy      93 

be  called  the  lateral  portion  of  the  nucleus  of  the  third 
nerve.  In  addition  to  these  there  is  yet  another  mass  of 
cells. 

(c)  In  the  center,  and  below  the  two  last  mentioned,  is 
a  cluster  of  larger  cells.  It  is  made  up  of  two  halves, 
which  coalesce  to  form  a  single  small  group  lying  in  the 
median  line.  It  is  spindle  form,  directed  up  and  forwards, 
only  two  or  three  millimeters  in  length,  and  is  composed  of 
cells  of  the  same  size,  form,  and  color  as  those  which,  on 
each  side,  we  already  know  as  a  and  a'.  Frontal  sections 
show  this  spindle-shaped  central  mass  to  be  bordered  with 
a  delicate  network  of  nerve  fibers.  Briefly  stated,  these 
are  the  principal  groups  of  cells  which  together  form  what 
is  known  as  the  nucleus  of  the  third  nerve,  namely,  the 
main  lateral  group  (a  and  a'),  the  smaller  group  (b  and  b') 
on  each  side,  and  the  central  group  in  the  median  line  (c). 
From  these  cells  the  fibers  pass  downwards  and  forwards, 
emerging,  as  is  well  known,  just  anterior  to  the  pons  near 
the  median  line. 

(B)  Cells  in  the  Cortex. — The  nucleus  just    described  is 
not,  however,  the  only  group  of  cells  sending  motor  fibers 
to  the  ocular  muscles.     We  know  at  least  one  other  point 
in  the  brain  in  which  such  cells  are  located,  and  there  may 
be  several.     Ferrier  noticed  that  irritation  of  a  certain  point 
of  the  cortex  of  the  frontal  lobe  was  followed  by  contrac- 
tions of  the  ocular  muscles,  and  his  experiments,  made  on 
monkeys,   have  been    verified    by   pathological    conditions 
found   in   man.      This  point   is  just   above   the  fissure  of 
Sylvius,  near  its  center,  as  shown  in  the  annexed  figure  of 
the  brain  of  a  monkey.     (Fig.  78.)     This  same  portion  is 
seen  in  Fig.  79.     We  have  still  much  to  learn  concerning 
the  general  structure,  arrangement  of  the  cells,  and  distribu- 
tion of  the  nerve  fibers  in  this  portion  of  the  brain.     In 
addition  to  the  cells  of  origin  (A)  in  the  nucleus  and  (B)  in 
the  cortex,  we  have 

(C)  The  Fibers   in  the  Brain    which    Connect  these   Two 
Points. 

There  is  no  question  as  to  the  existence  of  some  such 
fibers,  but  there  is  a  sad  lack  of  knowledge  of  their  number 


94      The  Third  Nerve — Microscopic  Anatomy 

and  direction  and  of  other  data  concerning  them.     With 
the  great  advances  made  by  Ramon  y  Cajal  and  others  in 


FIG.  78. — The  external  surface  of  the  right  half  of  the  brain  of  Macacus 
sinicus.  The  numbers  denote  the  chief  points,  the  excitation  of  which  evoked 
movements  of  the  eyes.  I.  Upward  movement  of  both  eyes.  2.  Downward 
movement  of  both  eyes.  3.  Movement  of  both  eyes  upward  and  to  the  oppo- 
site side.  4.  Movement  of  both  eyes  downward  and  to  the  opposite  side.  5. 
Convergence.  (Russell.) 


Gyrus  angttlarit 


Arteria 
fossae  Syhii 


FIG.  79. — View  of  the  gyrus  angularis  with  its  artery  (Bernheimer). 

methods  of  staining,  this  subject  seems  to  furnish  a  fruitful 
field  for  those  trained  in  microscopic  technique. 

It  has  seemed  worth  while  thus  to  devote  a  little  space  to 
the  deep  origin  of  this  nerve  because  of  its  own  importance, 


The  Third  Nerve — Macroscopic  Anatomy     95 


and  also  because  so  little  concerning  it  is  available  to  English 
readers. 

After  the  motor  oculi  leaves  the  brain,  it  is  not  only 
easily  followed,  but  is  so  well  known  that  it  is  simply  neces- 
sary to  turn  to  any  one  of  the  standard  works  of  anatomy 
or  of  ophthalmology 
to  find  there  a  de- 
tailed description  of 
its  course.  We  recall 
how  it  passes  forward 
(Fig.  80),  entering  the 
orbit  between  the  two 
heads  of  the  external 
rectus  muscle,  then 
almost  immediately 
separates  into  two  di- 
visions, one  of  which 
supplies  the  superior 
rectus  and  levator, 
while  the  inferior  divi- 
sion passes  to  the  inter- 
nal rectus,  and  to  the 
inferior  rectus  and  in- 
ferior oblique.  One  set 
of  the  branches  of  this 
nerve  is  particularly 
interesting,  namely, 

the  long  ciliary  nerves, 

7,  FIG.  80.— Third,  fourth,  and  fifth  nerves  in 

which  pierce  the  scle-    the  orbit     (Gray  } 

rotic  near  the  entrance 

of  the  optic  nerve,  passing  along  the  choroid,  to  be  dis- 
tributed to  the  iris  and  the  ciliary  muscle.  Their  importance 
in  connection  with  the  act  of  accommodation  is  evident. 
Another  set  of  branches  of  special  interest  are  those  in  the 
very  anterior  part  of  the  orbit,  which  anastomose  with 
branches  of  the  facialis.  It  is  not  impossible,  as  we  shall  see 
later,  that  upon  this  anastomosis  depends  the  contraction 
of  some  of  the  accessory  muscles  of  accommodation,  with 
the  frontal  headache  so  often  complained  of.  The  general 


Third  Nerve — Functions  of  the  Cells 


plan  of  distribution  of  the  main  branches  of  this  nerve  is 
shown  in  the  accompanying  diagram.     (Fig.  81.) 


FIG.  8l. — Plan  of  the  motor  oculi  nerve  (after  Flower). 

§  3.  Relation  of  Certain  Groups  of  Cells  in  the  Nucleus 
of  the  Third  Nerve  to  Certain  Ocular  Muscles. — In  this 
portion  of  our  study,  facts  relating  to  physiology  and 

pathology  are  ex- 
cluded as  much  as 
possible,  but  it  is 
best  at  this  point 
to  follow  still  one 
step    farther,    the 
between 
parts     of 


relation 
certain 

the  nucleus  and 
the  corresponding 
muscles,  although 
in  doing  so  we 
have  to  deal  ajso 
with  phases  of 
physiology.  Per- 
haps the  most  im- 
portant evidence 
on  this  point  is 
furnished  by  the 
experiments  of 
In  very  young  animals  he  destroyed 


FIG.  82. — Frontal  section  of  the  nucleus  of  the 
motor  oculi  of  a  rabbit.  In  this  animal  one  of  the 
third  nerves  was  destroyed  soon  after  birth.  When 
the  animal  was  grown  it  was  killed,  and  this  section 
made.  It  will  be  noticed,  that  the  nuclear  cells  on 
one  side  have  disappeared  (Von  Gudden,  Plate 
XXIX.,  Fig.  2). 

Von  Gudden  (B  133). 


Third  Nerve — Functions  of  the  Cells 


97 


separate  muscles  or  whole  groups  of  them,  drawing  out 
also,  in  some  cases,  the  nerves  which  supply  them;  then, 
when  the  animals  had  attained  maturity,  he  made  sections 
of  the  nucleus,  and  with  suitable  stains  ascertained  in  what 
part  of  the  nucleus  there  had  been  a  degeneration  of  the 
cells.  Some  of  his  findings  were  quite  striking.  (Fig.  82.) 


A    \    Tr.         flint 


FIG.  83. — Schematic  sagittal  section  through  the  nucleus  of  the  third  nerve 
(Bernheimer).  v,  anterior.  A,  posterior  portion.  The"  subdivisions  of  the 
nucleus  represent  the  grouping  of  the  cells  not  anatomically,  but  as  they  have 
been  determined  by  experiment,  with  more  or  less  exactness,  to  preside  over 
the  action  of  certain  muscles.  Thus  the  cells  in  the  anterior  group  control  the 
levator  palpebrse  ;  in  the  next  group,  the  rectus  superior  ;  then  the  rectus 
internus,  the  inferior  oblique,  and  the  rectus  inferior.  B.  M.  indicates  the 
location  of  cells  which  preside  over  the  sphincter  pupillse,  and  therefore 
probably  over  accommodation.  The  heavy  black  lines  from  these  cells  indi- 
cate that  the  fibers  go  only  to  the  muscle  which  is  shown  by  the  lettering. 
The  lighter  lines,  and  those  which  are  dotted,  indicate  that  the  fibers  go  to 
that  muscle  and  to  others  also. 

The  cells  included  on  the  dotted  space  posteriorly  ( Tr.}  are  not  a  portion 
of  the  nucleus  of  the  third  nerve.  This  is  the  nucleus  of  the  trochlearis. 

The  functions  of  the  various  groups  and  subdivisions  of 
groups  of  cells  which  constitute  the  nucleus  of  the  third 
nerve  have  been  investigated  by  different  observers  (B  131, 
J33»  l$6>  !6i,  165,  174,  etc.)  and  the  conclusions  arrived 
at  accord  in  the  main  so  well,  that  Bernheimer,  who  is 


98 


Third  Nerve — Functions  of  the  Cells 


probably  our  best  authority  on  this  point,  has  constructed 
the  accompanying  diagrams  to  show  which  portions  of  the 
cells  or  group  of  cells  preside  over  the  action  of  the  dif- 
ferent ocular  muscles.  (Figs.  83  and  84.)  In  these  dia- 
grams the  cen- 
tral portion, 
which  is  shad- 
ed, corresponds 
roughly  to  that 
group  of  cells 
which  in  the 
microscopic  sec- 
tions we  have 
called  (c).  The 
group  of  cells 
superiorly 
which  are  also 
shaded  are 
those  which  in 
the  sections  we 
have  called  b 
vand  b'.  The 
cells  and  parts 
of  the  groups 
which  are  not 
shaded 


FlG.  84. — Schematic  frontal  section  through  the  nu- 
cleus of  the  third  nerve  (Bernheimer).  The  same  as 
before,  except  that  this  shows  in  a  different  manner 
the  position  of  the  groups  of  cells  arranged  with  refer- 
ence to  their  physiological  action.  This  also  shows 
better  which  cells  have  fibers  that  cross  from  one  side 
of  the  brain  to  the  other. 


are 

those  which  cor- 
respond in  the 
frontal  sections 
to  those  groups  of  cells  which  we  have  called  a  and  a'. 
The  summary  of  our  knowledge  of  the  function  of  these 
different  groups  and  subdivisions  of  groups  may  be  stated 
about  as  follows. 

First.  The  group  of  cells  (c)  which  lies  in  the  median  line 
of  the  sections  gives  off  fibers  which  supply  the  ciliary 
muscles,  causing  contraction  of  the  pupil  and  assisting  in 
accommodation,  if  not  presiding  over  it. 

Second.  The  group  of  cells  lying  on  either  side  of  this 
band  also  presides  over  the  contraction  of  the  pupil.  It  is 


Third  Nerve — Functions  of  the  Cells         99 

not  certain  whether  each  of  the  fibers  of  these  two  groups 
passes  to  the  corresponding  side  of  the  body  or  whether 
they  are  crossed. 

Third.  The  group  of  cells  which  form,  on  either  side,  the 
principal  mass  of  the  nucleus  (a  and  a'  of  the  microscopic 
sections  or  the  parts  in  outline  of  the  diagram)  supplies,  as 
a  whole,  the  fibers  going  to  the  extraocular  muscles. 

These  three  points  are  established  with  a  very  con- 
siderable degree  of  certainty.  But  when  we  attempt 
to  go  beyond  that,  we  deal  with  probabilities  rather 
than  with  convincing  proof.  With  this  understanding,  we 
may  take  up  the  different  portions  of  these  lateral  groups  of 
cells. 

Fourth.  The  posterior  point — that  point  which  is  almost 
in  contact  with  the  nucleus  of  the  trochlearis — innervates 
the  inferior  rectus  muscle  of  the  opposite  side. 

Fifth.  The  cells  just  anterior  to  these  innervate  the  in- 
ferior oblique  of  the  opposite  side. 

Sixth.  The  cells  next  to  these  innervate  the  rectus 
internus. 

Seventh.  The  most  anterior  groups  innervate  the  superior 
rectus  and  the  levator,  although  this  and  the  preceding 
statement  are  hardly  more  than  inferences  by  exclusion. 

Such  is  a  general  outline  of  the  functions  of  these  different 
groups  of  cells  as  given  by  Bernheimer  and  others. 

While  making  this  digression  from  the  strictly  anatomical 
question  in  order  to  inquire  into  the  functions  of  these  groups 
of  cells,  we  may  include  the  clinical  evidence  bearing  on  this 
point.  Such  evidence  rests  upon  the  principle  that  if  two  or 
more  branches  of  the  nerve  (i.  e.,  muscles)  are  paralyzed, 
and  especially  if  these  two  or  more  are  affected  in  the 
same  or  in  a  similar  manner,  in  different  individuals,  then 
it  is  probable  that  the  cells  of  origin  of  these  branches  lie 
near  each  other.  If  the  number  of  such  cases  were  quite 
large,  it  would  thus  be  possible  to  construct  a  diagram 
showing  the  position  of  the  different  groups  of  the  cells 
with  regard  to  each  other,  as  indicated  by  these  partial 
paralyses.  Such  a  plan  has  been  elaborated  by  several  in- 
vestigators, especially  by  M.  Allen  Starr  (B  126)  of  New 


joo       Third  Nerve — Functions  of  the  Cells 


York.  (Fig.  85.)  For  this  purpose  he  collected  twenty 
cases  of  partial  paralysis  of  the  third  nerve,  and  arranged 
them  in  order  with  reference  to  the  "relative  position  of 

the  nuclei   and    the   ex- 

JCnAM*     fri,t,\  Citinru  i/IAtfJf 

tent  and  degree  of  par- 
alysis in  each  case  cited, ' ' 
the  numbers  in  his  dia- 
gram corresponding  to 
the  different  cases  cited 
in  his  list.  These  are  un- 
derlined when  the  par- 
alysis of  that  muscle  was 
complete.  It  is  interest- 
ing to  know  that  the  re- 
sults obtained  in  this  way 
clinically  correspond 
fairly  well  with  results 
obtained  by  experiments. 
If  our  cases  of  partial 
paralysis  of  the  third 
nerve  were  reported 
more  frequently,  we 
should  have  much  more 
abundant  data,  from 
which  conclusions  of 
value  might  be  drawn. 
Here  the  clinician  can 
aid  the  physiologist. 
The  student  who  ap- 


FIG.  85. — Diagram  by  M.  Allen  Starr  of 
cases  of  partial  paralysis  of  the  third  nerve. 


The  branches  which  have  been  affected  are     preaches  this  'subject  for 


placed  next  to  each  other,  and  for  conven- 
ience  they  are  arranged  as  if  all  were  on 
the  left  side.  The  numbers  are  simply  to 
identify  certain  cases  of  a  series  of  twenty, 
which  were  brought  together  by  Starr. 


the  first  time  may 
wonder  why  so  much  at- 
tention is  given  to  the 
relation  between  certain 
groups  of  cells  in  the 

nucleus  and  certain  ocular  muscles,  but  the  reason  will 
be  evident  enough  when  the  various  forms  of  paralysis  are 
considered. 

A  final  question,  partly  anatomical,  partly  physiological, 


Third  Nerve — Functions  of  the  Cells       101 

concerning  the  third  nerve,  is  its  connection  with  the 
occipito-frontalis,  the  corrugator  supercilii,  or  other  acces- 
sory muscles  of  accommodation,  for  in  the  chapters  on 
muscle  imbalance  we  shall  see  that  some  of  the  muscles 
controlled  by  the  third  nerve,  especially  the  internal  recti 
and  the  sphincter  of  the  iris,  bear  an  important  relation  to 
these  accessory  muscles.  Apparently  no  branch  of  the  third 
nerve  has  been  traced  to  those  muscles,  nor  is  there  any 
anastomosis  which  explains  entirely  the  physiological  and 
pathological  phenomena  with  which  we  are  familiar.  It 
should  be  remembered,  however,  that  the  terminal  fila- 
ments, and  especially  the  anastomosing  branches  of  the 
nerves,  are  often  microscopic  and  very  readily  mistaken  for 
connective  tissue  fibers.  No  one  appreciates  this  until  he 
has  worked  patiently  for  a  long  while  to  follow  a  single  fiber 
only  a  short  distance.  The  fact  that  we  can  not  readily  see 
such  an  anastomosis,  therefore,  does  not  prove  that  it  does 
not  exist.  The  intimate  relation  between  the  internal  rectus 
when  acting  with  the  ciliary  muscle  and  the  accessory 
muscles  of  accommodation  will  be  referred  to  in  the  chapter 
on  physiology,  and  in  the  part  relating  to  asthenopia. 

Corroborative  evidence  of  the  connection  between  these 
groups  of  muscles  is  shown  by  those  occasional  cases  of 
"total  ophthalmoplegia  in  which  the  occipito-frontalis  and 
orbicularis  palpebrarum  are  affected,  whilst  the  lower  facial 
muscles  escape."  (B  230.) 

One  of  the  most  important  observations  concerning  the 
nerve  supply  of  the  muscles  of  the  face  has  been  made  by 
Mendel  (B  226)  of  Berlin.  His  plan  of  study  was  similar  in 
principle  to  that  adopted  by  Von  Gudden,  and  the  conclu- 
sions, briefly  summed  up,  are  that  "the  frontal  and  orbicu- 
laris muscles,  although  peripherally  supplied  by  the  facial 
nerve,  are  'eye  muscles'  and  form  the  'oculo-facial '  group 
whose  central  innervation  is  the  oculo-motor  nucleus." 
This  gives  us  an  anatomical  basis  for  that  intimate  relation 
which  we  find  physiologically  and  clinically  between  efforts 
at  contraction  of  the  ciliary  muscles  and  of  the  accessory 
muscles  of  accommodation.  We  shall  see  later  that  the 
prolonged  and  forcible  contraction  of  these  accessory 


IO2  Fourth  Nerve 

muscles  causes  at  least  a  part  of  the  headache  and  discom- 
fort which  is  such  a  prominent  symptom  in  eye  strain. 

§  4.  Fourth  Nerve  (the  Trocnlearis). — The  deep  origin 
of  this  nerve  is  better  known  than  that  of  the  third,  and  its 
structure  is  less  complicated.  It  consists  of  a  single  group 
of  cells  on  each  side  of  the  brain  which  lie  posteriorly  and 
spinalward  from  the  cells  which  make  up  the  nucleus  of  the 
third  nerve.  The  nucleus  of  the  fourth  is  much  smaller 
than  that  of  the  third,  its  diameter  being  only  three  or  four 
millimeters,  or  large  enough  to  furnish  about  forty  moder- 
ately thick  sections.  It  is  roughly  hemispherical,  the  con- 
vex surface  being  directed  downward  and  backward.  When 
the  fibers  leave  this  nucleus  they  pursue  a  very  tortuous  and 


FIG.  86. — The  anastomoses  of  the  fourth  nerve  (Cl.  Bernard). 

unusual  course.  Starting  at  first  in  a  horizontal  direction 
backward,  they  turn  upward  in  front  of  the  motor  root  of 
the  trigeminus,  then  inwards  and  above  the  aqueduct  of 
Sylvius.  Emerging  from  the  brain  substance,  the  nerve 
crosses  to  the  opposite  side,  and  curves  forward  and  outward. 
Then,  being  in  contact  with  the  outer  side  of  the  crus,  it 
reaches  the  base  of  the  brain.  From  that  point  forward  its 
course,  like  that  of  the  other  cranial  nerves,  is  easy  to  fol- 
low. It  passes  along  the  edge  of  the  tentorium,  just  above 
the  opening  where  the  fifth  nerve  makes  its  exit  from  the 
skull,  and  enters  the  orbit  through  the  sphenoidal  fissure 
near  both  the  third  and  the  fifth  nerves.  Thence  it  turns 
inwards,  and  is  distributed  to  the  fibers  of  the  superior 
oblique.  The  anastomoses  of  the  fourth  nerve  are  shown  in 
the  accompanying  diagram  from  Claude  Bernard.  (Fig.  86.) 


Fifth  Nerve 


103 


§  5.  Fifth  Nerve  (the  Trigeminus). — The  cells  at  the 
origin  of  the  two  roots  of  this  nerve  are  arranged  principally 
in  a  long  row  which  extends  on  each  side  of  the  median  line 
of  the  medulla.  (Fig.  73.)  Commencing  above,  on  either 
side,  in  the  substance  of  the  corpora  quadrigemina,  they 
stretch  downward  and  backward  for  some  three  centimeters 
or  more.  Opposite  the  center  of  the  pons  the  cells  are 
especially  abundant,  and  the  fibers  passing  basalwards  have 
the  appearance,  when  taken  together,  of  a  short,  thick 
camel's-hair  brush,  with  the  point  held  upwards.  From 
this  group  of  cells,  or  principal  nucleus  of  the  fifth,  as  it 
may  be  called,  another  long  band  of  cells  stretches  off 
spinalwards  at  the  side  of  the  median  line.  As  the  nerve 
fibers  from  each  cell  pass  first  to  the  principal  nucleus,  to 
join  with  the  other  fibers,  this  extension  backward  of  the 
cells  with  their  fibers  looks  like  a  single  line.  The  cells 
which  belong  to  the  sensitive  fibers  are  small  and  round 
like  those  of  the  sensitive  roots  in  the  spinal  cord.  These 
cells  form  most  of  the  principal  nucleus,  and  of  the  row 
which  stretches  from  that  point  spinalward. 

The  cells  which  belong  to  the  motor  root  are  large  and 
multipolar.  They  form  only  a  small  part  of  the  principal 
nucleus,  and  extend  from  that 
point  cerebralwards  to  the  cor- 
pora quadrigemina,  as  already 
stated.  The  fibers  of  this  group 
of  cells  also  are  directed  first  to- 
ward the  principal  nucleus  and 
from  that  point  all  of  the  parts 
of  the  motor  root  join  to  emerge 
near  the  middle  of  the  pons  on 
each  side  of  the  median  line. 

As  the  sensitive  and  motor 
roots  thus  leave  the  brain  to- 

,1  ,,         g  FIG.  87. — Frontal  section  of  the 

gether,    the    former    soon    en-        .     .  ' 

orbit  with  recti  muscles  showing  the 

larges    into    the    well  -  known  entrance  into  the  orbit  of  the  third, 

Gasserian     or     semilunar     gan-    fourth,  and  fifth  nerves  (Merkel). 

glion,  and  then  separates  into 

its  three  portions, —  the  ophthalmic,  the  superior,  and  the 


IO4 


Fifth  Nerve 


inferior  maxillary  branches.  The  first  of  these,  passing 
through  the  sphenoidal  fissure  (Fig.  87),  divides  into  the 
lacrymal,  frontal,  and  nasal  nerves,  as  figured*  in  most  text- 


InltrnaZ  ffanfj  At. 
&  Camt 


FIG.  88. — Fifth  and  sixth  nerves  in  the  orbit  (Gray). 


FIG.  89. — Plan  of  part  of  the  fifth  nerve  (Flower). 


Sixth    Nerve 


105 


books  and  as  seen  in  Fig.  88.  The  fact,  however,  which  is 
probably  of  the  most  importance  in  this  connection  is  that 
the  terminal  fibers  of  the  two  latter  subdivisions,  as  they 
spread  out  upon  the  forehead,  supply  the  corrugator  super- 
cilii,  the  occipito-frontalis,  and  other  muscles  in  this  vicinity, 
— that  is,  the  accessory  muscles  of  accommodation.  (Fig. 
89.)  The  bearing  of  this  fact  will  be  seen  when  we  consider 
questions  relating  to  ocular  headaches.  We  should  also 
note  that  anastomoses  occur  between  the  smaller  branches 
of  the  ophthalmic  and  the  superior  maxillary  nerves,  as 
this  may  account  for  pain  in  and  about  the  eyes  sometimes 
associated  with  dental  caries. 

§  6.  Sixth  Nerve  (the  Abducens).— The  group  of  multi- 
polar  ganglion 

cells  which  consti- 
tute the  deep  ori- 
gin of  this  nerve 
is  situated  just  be- 
neath the  floor  of 
the  fourth  ventri- 
cle, almost  exactly 
in  its  center  from 
before  backward 
and  two  or  three 
millimeters  from 
the  median  line. 
They  may  be  apt- 
ly compared,  like 
the  principal 

group  of  the  nucleus  of  the  fifth,  to  the  tip  end  of  a  brush, 
two  or  three  millimeters  in  diameter.  (Figs.  90  and  .91.) 
From  this  point  the  fibers  pass  straight  downward  and  out- 
ward toward  their  point  of  exit  from  the  brain.  At  first  a 
few  fibers  joined  to  each  other  form  separate  bands,  but 
these  unite  before  they  emerge  from  the  brain. 

It  is  interesting  to  observe  the  curious  relation  of  the 
seventh  nerve  to  the  sixth,  near  their  points  of  origin  (Fig. 
90). 

The  sixth,   as  just  mentioned,  commences  in  a  brush- 


FIG.  90. — Relation  of  the  nuclei  near  the  origin 
of  the  sixth  and  seventh  nerves  (Mauthner). 


io6 


Sixth  Nerve 


shaped  extremity  and  passes  straight  down  and  outward  to 
its  destination.  The  nucleus  of  the  seventh  is  at  first  be- 
low, spinalwards  and  laterally  from  that  of  the  sixth.  From 
that  point  the  fibers  start  first  dorsal  and  cerebralward  and, 
curving  almost  in  a  loop  around  the  nucleus  of  the  sixth, 
retrace  their  course  and  pass  downwards  and  outwards  in  a 
direction  similar  to  that  of  the  sixth  nerve.  Indeed,  in  the 
latter  part  of  their  course  in  the  brain  the  sixth  and  seventh 
lie  very  nearly  in  the  same  frontal  plane  (Fig.  91),  the  latter 


FIG.  91. — Frontal  section  through  the  origin  of  abducens  nerve  (Edinger). 

nerve  emerging  somewhat  farther  than  the  sixth  from  the 
median  line.  Neither  the  sixth  nor  the  seventh  crosses 
from  one  side  of  the  brain  to  the  other. 

On  emerging  from  the  brain,  the  sixth  nerve  passes  along 
the  groove  on  one  side  of  the  sphenoid  bone,  enters  the 
orbit  between  the  two  heads  of  the  external  rectus  muscle, 
and  at  once  divides  into  small  filaments  which  are  distributed 
to  the  fibers  of  that  muscle. 

§  /.  Branches  of  the  Sympathetic. — Any  description  of 
the  nerve  supply  to  the  ocular  muscles  would  be  incom- 
plete without  some  mention  of  the  innervation  through  the 


Branches  of  the  Sympathetic  107 

sympathetic.  These  branches  come,  as  we  know,  from  the 
ophthalmic  ganglion.  This  is  usually  described  as  a  flat- 
tened lenticular  bit  of  reddish  gray  matter,  two  or  three 
millimeters  long,  lying  at  the  back  part  of  the  orbit  just  ex- 
ternal to  the  optic  nerve  (Fig.  88).  Some  descriptions  give  it 
three  afferent  branches,  and  others  describe  only  two.  The 
longer  or  superior  branch  is  well  marked,  and  comes  from 
the  nasal  branch  of  the  ophthalmic  nerve.  A  filament  also 
comes  from  the  cavernous  plexus  and  this  is  sometimes 
joined  by  another  afferent  branch,  which  comes  from  the 
twig  going  to  the  inferior  oblique.  The  branches  of  distri- 
bution from  the  ophthalmic  ganglion  vary  in  number  from 
three  to  half  a  dozen.  All  of  these  pierce  the  sclerotic  not 
far  from  the  entrance  of  the  optic  nerve,  and  pass  forward 
ultimately  to  the  ciliary  process  and  to  the  iris. 

Only  one  who  has  attempted  to  make  a  dissection  of  this 
ganglion  can  appreciate  why  the  descriptions  given  of  it 
vary  so  decidedly.  Even  when  one  knows  exactly  where 
the  ganglion  lies  it  is  difficult  to  find  it,  and  after  it  is  found, 
it  is  almost  impossible  to  decide  whether  the  hair-like  fila- 
ments which  lead  to  and  from  it  are  really  nerves  or  only 
fine  threads  of  connective  tissue.  In  any  case  they  are  very 
apt  to  be  torn  before  the  dissection  is  completed.  The 
ganglion  is  rarely  seen  in  anatomical  collections,  although  a 
very  beautiful  dissection  was  shown  me  some  years  ago  by 
Axenfeld  of  Freiburg. 

As  mention  has  been  made  of  the  functions  of  the  cells 
in  the  nucleus  of  the  third  nerve,  so  in  passing  we  should 
make  note  here  of  the  action  of  the  branches  of  the  sym- 
pathetic. It  is  generally  supposed  that  they  control  the 
action  of  the  dilator  pupillae.  While  this  is  one  of  their 
functions  it  is  probably  not  the  only  one.  Indeed,  the 
study  of  the  anatomy  and  physiology  of  the  ophthalmic 
ganglion  —  to  say  nothing  of  its  pathology  —  is  a  subject 
which  has  been  too  much  overlooked,  and  judging  from  the 
paucity  of  literature  on  that  subject  it  is  an  excellent  one 
for  investigation. 


CHAPTER  IV. 


THE  BLOOD-VESSELS. 


THE  blood-vessels  which  supply  the  muscles  are  not  of 
sufficient  importance  to  warrant  more  than  passing  notice. 
The  accompanying  illustration  (Fig.  92)  shows  that  as  the 

ophthalmic  ar- 
tery enters  the 
orbit,  it  divides 
into  various 
branches,  some 
going  to  the 
muscles  and 
some  to  the 
globe.  It  is 
worth  while  to 
recall  a  point 
concerning  the 
arteries  distrib- 
uted to  the  cili- 
ary muscle.  Of 
its  three  sources 
of  supply,  one 
is  the  anterior 
short  arteries. 

•r~t  r««a*  These  consist  of 

minute  branches 
from  the  recti, 
near  their  inser- 
tion, and  from  that  vicinity.  They  pass  over  the  globe, 
perforating  it  at  a  short  distance  from  the  margin  of  the 
cornea.  This  apparently  accounts  for  the  fact  that  any 
change  in  the  blood  supply  to  the  extraocular  muscles  may 
influence  that  of  the  intraocular  muscles  also. 

108 


FlG.  92. — Blood  supply  of  the  muscles  (Gray). 


The  Blood- Vessels  109 

There  is  one  small  artery,  though,  of  special  interest  to 
the  student  of  the  muscles.  That  branch,  however,  is  not 
near  the  muscles,  but  in  the  brain.  It  is  the  arteria  fossae 
Sylvii  with  the  accompanying  vein.  (Fig.  79.)  After  this 
artery  passes  upward  and  backward  along  the  fossa  of 
Sylvius  it  divides  usually  into  three  branches.  One  of 
these  passes  down  and  backwards,  another  almost  directly 
upwards,  while  the  central  branch,  which  in  most  brains  is 
practically  a  continuation  of  the  original  artery,  extends 
backwards  to  supply  the  gyrus  angularis.  It  is  in  this  im- 
mediate vicinity,  as  we  have  already  seen,  that  the  cortical 
cells  are  situated  which  also  control  the  movements  of 
the  muscles.  In  the  part  of  this  study  which  relates  to 
pathological  conditions  we  shall  find  that  certain  forms  of 
paralyses  are  so  sudden  in  their  onset  as  to  indicate  beyond 
doubt  that  they  are  due  to  an  effusion  of  blood.  Such  a 
history,  together  with  other  clinical  evidence,  as  well  as 
post-mortem  conditions,  shows  that  in  a  considerable  pro- 
portion of  these  cases  the  hemorrhage  is  not  in  the  vicinity 
of  the  nucleus  of  the  third  nerve,  but  in  the  region  of  the 
gyrus  angularis — in  other  words,  from  some  branch  of  this 
arteria  fossae  Sylvii. 


CHAPTER  V. 

COMPARATIVE   ANATOMY   AND   EMBRYOLOGY   OF   THE 
OCULAR  MUSCLES. 

§  i.  Ocular  Muscles  of  the   Lower  Vertebrates.— A 

glance  at  the  eye  muscles  of  the  lower  vertebrates  shows 
that  their  general  arrangement  is  similar  to  that  which  exists 
in  the  human  orbit,  except  that  we  often  find  in  addition, 
especially  among  the  mammals,  a  strong  muscle  known  as 
the  retractor  or  choanoid  muscle.  A  great  difference  in  the 
details  of  their  arrangement,  however,  exists  among  the 
different  orders  and  genera.  At  the  outset  we  should  re- 
member that  as  the  position  of  the  animal's  head  is  usually 
horizontal,  the  terms  indicating  direction — up  and  down, 
in  and  out — have  a  different  meaning  than  when  used  with 
reference  to  human  anatomy.  Thus  "posterior"  indicates 
the  direction  from  the  anterior  to  the  posterior  portion  of  the 
animal,  although  in  some  instances  when  the  head  inclines 
downward  this  means  really  from  below  upward.  Also,  the 
term  "inwards"  means  toward  the  median  line,  although  in 
some  cases  this  becomes  upward ;  outwards  is  of  course  in 
the  opposite  direction. 

Dissection. — In  dissecting  the  ocular  muscles  of  the'fishes 
and  even  some  of  the  mammals  it  is  better  to  disarticulate 
the  lower  jaw  and  open  the  orbit  from  the  roof  of  the  mouth. 
In  this  way  the  best  view  is  usually  obtained,  especially 
in  the  fishes,  as  with  them  the  ocular  muscles  extend  from 
the  sphenoidal  canal.  With  the  higher  mammals  like  oxen, 
hogs,  dogs,  etc.,'  it  is  better  to  remove  the  skullcap  and 
approach  through  the  orbit  from  above,  as  in  dissecting  the 
human  eye. 

The  literature  of  the  comparative  anatomy  of  the 
ocular  muscles  is  not  extensive.  An  excellent  descrip- 

IIO 


Comparative  Anatomy  1 1 1 

tion  was  given  of  the  muscles  of  the  cat  by  Mivart  in 
1 88 1,  and  in  the  monograph  by  Motais  (B  17)  already  re- 
ferred to  the  subject  is  treated  at  considerable  length. 
He  has  made  beautiful  dissections  of  the  eye  muscles  of 
different  animals,  and  I  have  been  able  to  verify  the  de- 
scriptions as  regards  several  of  the  common  fishes,  the  do- 
mesticated birds — such  as  thec  hicken  and  turkey, —  and 
the  larger  domesticated  animals,  including  the  pig,  horse, 
sheep,  and  ox. 

Fishes. — Among  the  fishes  of  the  lower  types  or  the 
Chondropterygiens  (those  having  cartilaginous  coverings) 
we  find,  instead  of  six  muscles,  that  the  recti  are  bifurcated 
or  subdivided  into  smaller  bands,  so  as  to  give  the  appear- 
ance of  a  larger  number  of  muscles — as,  for  example,  is 
shown  in  the  sunfish  (Orgathoriscida  mold).  This,  perhaps, 
is  a  remnant  of  the  still  earlier  forms  of  life  in  which  the 
eye  is  moved  either  by  one  cone  of  muscles,  or  by  many 
small  filaments  attached  to  different  parts  of  the  globe. 
Even  in  this  class  of  the  lower  fishes  we  find  in  some  in- 
stances that  the  globe  is  turned  by  four  well-marked  recti 
muscles  and  two  obliques,  although  there  are  individual 
differences  in  this  respect. 

Passing  to  the  fishes  with  bony  skeleton,  or  the  Teleos- 
teins,  we  find  still  more  frequently  the  type  of  the  four  recti 
muscles  with  the  two  obliques,  the  recti  usually  passing  out- 
wards and  forwards  from  the  optic  foramen.  The  superior 
and  the  inferior  oblique,  however,  spring  from  the  more  an- 
terior portion  of  the  skull  just  above  the  roof  of  the  mouth, 
near  its  front  portion,  and  then  pass  backward.  Occasion- 
ally, among  these  fishes — as,  for  example,  in  the  mackerel, 
and,  to  some  degree,  in  the  ordinary  whitefish  (Coregonus 
albus) — there  is  an  ingenious  contrivance  for  increasing  the 
length  and  therefore  the  action  of  the  recti  muscles.  Care- 
ful dissection  shows  that  a  small  canal  —  the  sphenoidal 
— extends  from  the  orbit  directly  backward  near  the  median 
line  just  above  the  roof  of  the  mouth.  This  canal  is  prac- 
tically an  extension  of  the  orbit  almost  at  right  angles  to  its 
principal  axis.  The  recti  muscles  arise  either  from  the  ex- 
treme end  of  this  canal  or  along  its  side,  and  then,  passing 


1 1 2  Comparative  Anatomy 

forward  into  the  orbit  proper,  they  turn  outward  at  a  more 
or  less  sharp  angle  to  be  inserted  into  the  globe.  This  ar- 
rangement, of  course,  adds  much  to  the  efficiency  of  the 
muscles. 

Amphibians  and  Reptiles. — Here,  again,  we  find  the  same 
tendency  to  division  into  four  recti  and  two  obliques,  and 
in  addition  there  appears  in  some  a  more  or  less  marked  re- 
tractor muscle  of  the  globe.  This  arises  immediately  behind 
the  globe,  and,  passing  outward,  is  attached  into  the  scler- 
otic near  the  optic  nerve.  It  is  almost  pyramidal  in  shape, 
the  apex  being  situated  at  the  origin  of  the  muscles,  with 
the  base  resting  on  the  globe,  between  the  insertions  of  the 
recti  muscles  and  the  optic  nerve. 

In  a  word,  among  these  animals  also,  although  the  number 
and  position  of  the  eye  muscles  differ  greatly,  there  is  the 
same  general  arrangement  as  in  the  orbit  of  the  fishes, 
except  that  there  is  seldom  an  extension  of  the  recti  into 
the  sphenoidal  canal.  A  curious  muscular  arrangement 
is  met  with,  which,  though  not  belonging  strictly  to  the 
motor  muscles,  is  worthy  of  mention,  as  it  occurs  also  in 
many  of  the  higher  types,  especially  among  the  birds,  cats, 
etc.  This  is  the  tensor  membrana  nictitans.  Among  the 
reptiles,  this  muscle  is  contained  in  a  rather  long  tube  situ- 
ated at  the  outer  or  posterior  angle  of  the  eye  and  is  at- 
tached by  a  minute  ligament  to  the  third  lid.  A  contraction 
of  this  muscle  causes  this  third  lid  to  sweep  entirely  across 
the  globe. 

Birds. — Here  is  again  the  same  general  plan  of  six  muscles 
for  the  movement  of  the  globe,  although  there  are  often 
two  others  for  the  third  lid.  The  plan  of  the  origin  and  in- 
sertion of  the  recti  and  of  the  obliques  is  also  in  general  the 
one  with  which  we  are  familiar.  Among  the  birds  the 
retractor  bulbi  is  seldom  found.  A  very  ingenious  and 
curious  arrangement,  described  more  than  a  hundred  years 
ago  by  Petit,  Hunter,  and  others,  ensures  the  rapid  move- 
ment of  the  third  lid. 

With  the  birds  this  third  lid  has  its  origin,  as  usual,  near 
the  inner  canthus,  being  somewhat  quadrilateral  in  shape. 
To  its  outer  and  upper  edges  a  fine  tendinous  filament  is 


Comparative  Anatomy  1 1 3 

attached,  which  passes  up  and  outward  around  the  equator 
of  the  eye,  then  behind  the  globe,  and  winding  around 
backwards  and  downwards  around  the  optic  nerve,  it  termi- 
nates in  a  muscular  band  behind  the  globe.  Moreover,  in 
its  course  around  the  optic  nerve  it  is  drawn  away  from  it 
by  still  another  muscular  band,  through  a  loop  which  holds 
it  in  place.  (B  244,  vol.  ii.,  p.  143.)  In  this  way  very  much 
is  added  to  the  power  of  the  muscle,  with  corresponding 
economy  of  space. 

Mammals. — It  is  natural  to  expect  that  in  this  class  the  re- 
semblance of  the  orbit  and  the  ocular  muscles  to  those  of 
the  human  species  would  be  closer  and  more  constant  than 
in  any  other  class.  That,  however,  is  not  always  the  case. 
A  glance  at  the  skeletons  of  the  whales,  which  are  so  com- 
mon in  museums,  shows  that  part  of  the  walls  of  the  orbit 
is  entirely  lacking.  We  find,  too,  that  the  orbits  of  the 
rodents  are  small,  and  in  several  other  families  the  orbital 
walls  are  so  arranged  as  to  restrict  necessarily  the  free  action 
of  ocular  muscles.  In  general,  though,  these  muscles  are 
arranged  on  the  same  plan  as  in  man  and  the  lower  animals, 
namely,  four  recti  and  two  obliques,  while  many  possess 
also  the  choanoid  or  the  retractor  bulbi. 

It  is  unnecessary  for  our  present  purpose  to  go  into  details 
concerning  the  arrangement  of  the  ocular  muscles  in  the 
different  orders  of  the  mammals.  As  to  the  recti,  it  is 
worth  while  to  observe  that  inasmuch  as  they  do  not  usually 
spring  from  around  the  optic  nerve,  as  in  the  human  subject, 
but  rather  from  one  side  of  the  orbit,  the  axis  of  the  cone 
of  these  muscles  is  therefore  at  an  angle  more  or  less  acute 
to  the  axis  of  the  globe. 

The  retractor  muscle  is  found  in  some  of  the  highest  and 
also  most  of  the  lowest  groups  of  the  mammals.  The  eyes 
of  the  hog  and  of  the  horse  furnish  very  well-marked  and 
familiar  examples.  In  certain  varieties,  the  muscle  which 
moves  the  third  lid  is  a  prominent  and  important  portion  of 
the  ocular  anatomy,  and  the  possible  relation  of  this  to  the 
muscle  of  Horner  has  already  been  noted. 

A  word  may  be  added  concerning  the  action  of  the  ocular 
muscles  in  the  lower  animals ;  the  function  of  the  recti  and 


H4  Embryology 

of  the  obliques  is  of  course  the  same  as  in  man;  the  re- 
tractor bulbi  moves  the  globe  also  in  different  directions,  as 
do  the  rectij  but  to  a  limited  extent.  The  function  of  this 
muscle,  though,  is  pre-eminently  the  protection  of  the  globe 
by  drawing  it  farther  within  the  orbit.  This  can  be  easily 
seen  in  the  horse  by  tapping  on  the  closed  lid. 

On  one  occasion  when  making  an  experimental  opera- 
tion on  the  eye  of  a  horse,  after  the  chloroform  had  been 
administered  to  a  point  which  was  considered  sufficient,  a 
suitable  speculum  was  introduced  between  the  lids.  On 
attempting  to  fix  the  globe,  however,  the  horse  drew  the 
globe  so  far  into  the  socket  as  to  make  the  operation  im- 
possible; in  fact,  only  a  small  part  of  the  cornea  remained 
visible.  But  when  more  chloroform  was  administered  and 
the  retractor  bulbi  consequently  relaxed,  the  globe  came 
forward  to  its  usual  position. 

It  would  be  interesting  to  know  whether  an  analogue  of 
the  retractor  bulbi  exists  even  occasionally  in  the  human 
subject  in  the  form  of  a  supernumerary  muscle.  I  have 
never  happened  to  see  any  trace  of  this  when  making  dis- 
sections of  the  orbit,  nor  is  it  mentioned  by  Bochdalek. 
It  is  certain,  however,  that  in  rare  instances  individuals  are 
able  to  retract  the  globe  within  the  orbit,  and  cases  of 
voluntary  retraction  reported  by  Axenfeld  suggest  that 
these  persons  either  have  certain  fibers  similar  to  the  re- 
tractor bulbi  of  the  lower  animals,  or  else  that  the  recti 
muscles  act  in  a  very  unusual  manner  to  produce  that 
result. 

§  2.  Embryology. — As  our  knowledge  of  embryology  in- 
creases we  find  constantly  a  larger  number  of  facts  which 
explain  our  clinical  experiences.  Therefore,  aside  from  any 
general  scientific  interest  which  the  subject  may  have,  it  is 
well  to  glance  at  a  few  points  connected  with  the  foetal  de- 
velopment of  the  ocular  muscles.  It  is  only  possible  to  give 
here  an  outline  of  what  is  found  in  detail  in  the  admirable 
articles  of  Ryder  (B  248)  and  others.  Frequently  when 
studying  this  subject  it  is  convenient  to  make  use  of  the  pig, 
and  as  such  observations  can  be  readily  verified,  this  general 
description  refers  particularly  to  th£.t  animal.  When  the 


Embryology  115 

embryo  is  about  twenty  days  old  it  measures  some  ten  milli- 
meters in  length,  the  head  or  anterior  portion  being  about 
four  millimeters  long.  It  is  much  bent  on  itself,  and  has  a 
slightly  spiral  form.  The  lower  portion  of  the  head  is  then 
hardly  distinguishable,  but  already  the  vesicle  of  the  eye 
has  begun  to  form,  and  behind  it  there  is  a  nucleus  from 
which  the  muscles  develop.  About  that  time  the  first  trace 
of  the  third  and  sixth  nerves  appears,  but  the  fourth  cannot 
then  be  discovered  (B  246). 

When  the  embryo  has  reached  fifteen  or  twenty  milli. 
meters  in  length  the  development  of  the  ocular  muscles 
has  increased  even  more  rapidly.  By  that  time  the  superior 
oblique  and  superior  rectus  are  well  defined,  being  quite 
closely  joined  together.  The  inferior  rectus  and  inferior 
oblique  are  also  distinguishable,  lying  near  to  each  other. 
The  external  rectus  is  represented  by  a  small  projecting 
point,  while  the  internal  is  not  apparent  in  these  sections. 
The  third  nerve,  however,  by  that  time  is  well  marked. 

Still  later,  when  the  embryo  has  grown  to  be  about  half 
as  large  again,  the  extraocular  muscles  are  well  defined  and 
easily  found  by  dissection.  Their  relative  position  also 
is  almost  the  same  as  that  which  they  occupy  in  adult  life; 
in  other  words,  these  external  muscles  are  all  formed  at 
quite  an  early  s^age.  For  it  should  be  understood  that  at 
this  stage  the  eye  itself  is  far  from  complete.  The  lens 
is  still  in  contact  with  the  cornea,  the  iris  has  not  yet  ap- 
peared as  a  distinct  structure,  there  is  no  anterior  chamber, 
and  no  true  cornea.  The  lids  at  that  time  are  represented 
only  by  rudimentary  folds  of  skin  above  and  below  the 
globe.  Marshall  (B  247,  page  296)  calls  special  attention 
to  the  fact  that  the  external  rectus  has  "nothing  whatever 
to  do  with  the  first  head  cavity,  though  it  ultimately 
reaches  the  eyeball."  He  further  says:  "This  is  a  point 
whose  importance  can  hardly  be  overrated,  as  it  furnishes 
us  with  an  explanation  of  the  fact  that  the  rectus  externus 
is  supplied,  not  by  the  third  nerve,  but  by  a  totally  distinct 
nerve — the  sixth."  Of  the  four  muscles  which  are  supplied 
by  the  third,  three  of  them,  and  the  fourth  possibly,  are 
developed  from  the  walls  of  the  first  head  cavity. 


n6  Embryology 

The  subsequent  history  of  the  muscles  of  the  embryo 
leaves  but  little  to  describe.  From  the  stage  last  referred 
to,  until  birth,  the  development  consists  principally  in  an 
increase  of  the  muscle  tissue,  the  arrangement  and  relative 
size  of  the  muscles  remaining  unchanged. 

No  consideration  of  the  development  of  the  muscles  of 
the  eye,  however  cursory,  would  be  complete  without  hav- 
ing attention  directed  to  the  development  of  the  nerves 
which  supply  these  muscles.  The  third  nerve,  as  we  have 
mentioned,  appears  at  a  very  early  stage,  but  it  soon  divides 
into  a  dorsal  and  a  ventral  branch,  as  Corning  (B  246)  pointed 
out.  An  important  point  in  connection  with  this  nerve,  and 
perhaps  one  of  the  most  important  facts  in  the  embryology 
of  the  eye,  is  that  the  foetal  third  nerve  supplies  not  only  the 
four  eye  muscles,  but  also,  in  certain  forms,  the  branches 
of  this  nerve,  extending  outward,  supply  part  of  the  tissues 
which  later  become  accessory  muscles  of  accommodation. 
This  gives  us  another  clue  to  the  important  relation  between 
the  muscles  in  the  orbit  and  those  which  have  been  called  by 
Mendel  (B  226)  the  "oculo-facial  "  group.  The  fourth  nerve 
is  developed  quite  independently  of  the  third.  It  begins  ap- 
parently as  a  portion  of  the  ganglion  of  the  fifth  nerve,  but 
the  intermediate  stages  of  its  development  are  as  yet  not 
clearly  understood.  The  sixth,  like  the  motor  oculi,  must 
be  considered  as  coming  from  the  ventral  surface,  or  at  least 
it  has  what  we  call  a  ventral  root.  In  a  word,  we  have  the 
third  nerve  with  the  muscles  to  which  it  is  distributed  com- 
ing from  one  group  of  cells,  the  sixth  nerve  with  the  muscle 
to  which  it  is  distributed  coming  from  quite  another  group 
of  cells,  while  the  fourth  nerve  is  at  first  apparently  a  part 
of  the  fifth,  but  later  quite  independent  of  it,  and  goes  to 
a  muscle  which  is  most  nearly  related  to  the  group  supplied 
by  the  third  nerve. 


PART  II. 
PHYSIOLOGY. 

CHAPTER  I. 
ONE   EYE  AT  REST. 

§  i.     Introduction  to  the  Physiology  of  the  Muscles. — 

The  "  practical  ophthalmologist  "  may  perhaps  think  it  quite 
unnecessary  to  review  the  physiology  of  the  muscles,  for  the 
reason  that  this  phase  of  the  subject  has  been  thoroughly 
worked  out  already.  In  the  main  that  is  quite  true,  and  the 
files  of  Gracfes  Archives  and  other  journals  of  that  class 
show  what  careful  studies  have  been  made  of  the  normal 
ocular  movements.  But  it  is  also  true  that  many  of 
the  facts  there  recorded  have  no  practical  significance  which 
can  be  seen  in  the  light  of  our  present  knowledge.  It 
seems  proper  therefore  to  select  from  this  mass  of 
observations  those  which  are  apparently  of  clinical  im- 
portance, and  arrange  them,  if  possible,  in  such  a  manner 
as  to  form  a  systematic  basis  for  clinical  work.  Moreover,  it 
is  better  thus  to  bring  the  physiological  data  together  than 
to  scatter  them  through  chapters  where  they  would 
necessarily  be  confused  with  what  relates  to  pathology. 

When  dealing  with  these  questions  we  are  obliged 
to  turn  almost  constantly  to  those  Continental  author- 
ities, especially  the  Teutons,  who  laid  the  foundations 
of  our  science  with  mathematical  exactness.  But  when  we 
come  to  the  clinical  conclusions  resting  on  those  foundations 
we  shall  have  more  to  do  with  the  applications  of  Anglo- 
Saxon  ingenuity.  If  one  may  venture  to  follow  the  exam- 
ple of  Tyndall  and  compare  the  subject  of  our  study  to  a 

"7 


n8  Plan  of  Study 

geometrical  figure,  our  knowledge  of  the  ocular  muscles 
could  be  represented  graphically  by  a  pyramid.  It  might 
be  said  that  the  one  constructed  by  the  earlier  physiologists 
and  mathematicians  had  a  broad  base,  but  did  not  reach  far 
upward  toward  practical  conclusions.  On  the  other  hand,  in 
much  that  is  written  to-day,  the  effort  to  attain  the  practi- 
cal end  immediately,  makes  the  pyramid  too  high  for  its 
base.  Sometimes,  indeed,  the  pyramid  seems  inverted. 
The  problem  is,  therefore,  how  to  make  the  structure  strong 
and  of  goodly  proportions. 

Our  first  step  must  be  to  agree  upon  definitions,  and  for 
this  purpose  to  view  the  eye  as  a  globe  at  rest,  examine 
its  different  planes,  axes,  and  the  angles  which  they  form 
with  each  other,  as  reference  must  be  made  to  these 
almost  constantly.  Then  it  is  advisable  to  study  the  action 
of  the  ciliary  muscle  and  how  accommodation  is  affected 
by  cycloplegics  or  myotics.  The  special  reason  for  follow- 
ing this  order,  is  that  as  soon  as  we  approach  any  of  the 
questions  which  relate  to  muscle  imbalance,  we  must 
deal  first  of  all  with  the  ciliary  muscle  by  seeking  to  bring  it 
into  normal  relation  to  the  extraocular  muscles.  After  de- 
ciding what  we  are  to  understand  by  "accommodation,"  we 
can  study  the  ocular  motions.  The  movements  of  the 
globe  are  somewhat  complicated,  it  is  true,  but  the 
difficulty  in  understanding  them  will  be  greatly  lessened 
if,  beginning  with  one  eye  only,  we  observe  first  the  sim- 
plest motion  which  it  can  make, — in  and  out,  up  and  down, — 
and  the  limits  of  these  motions.  A  further  study  of  the 
lateral  motions  will  lead  us  to  consider  the  amount  of  force 
which  the  muscles  can  exert  in  making  them,  the  time 
necessary  to  accomplish  this  and  how  to  measure  it,  both 
when  the  globe  swings  uninterruptedly  through  an  arc  of  a 
certain  length,  or  when  it  goes  halting,  as  in  the  act  of 
reading.  All  these  motions  are  comparatively  simple  and 
can  be  made  by  the  rotation  of  the  globe  about  an 
axis,  either  horizontal  or  vertical.  After  understanding 
these,  we  will  be  better  prepared  to  consider  other  rotations, 
and  the  laws  which  govern  them,  especially  those  relating 
to  convergence  and  torsion 


Plan  of  Study  119 

Indeed,  it  may  be  stated  at  the  outset,  that  the  object  of 
the  physiological  part  of  this  study  is  not  simply  to  review 
facts  already  known,  or  perhaps  to  add  a  few  which  are  new, 
but  it  is  rather  to  proceed  in  such  a  manner  that  we  may  be 
gradually  led  to  a  conclusion  which  has  a  most  important 
significance.  This  is,  that  FOR  COMFORTABLE  BINOCULAR 
VISION,  ESPECIALLY  AT  THE  WORKING  DISTANCE,  A  RELA- 
TION WITHIN  CERTAIN  LIMITS  MUST  BE  MAINTAINED  BE- 
TWEEN ACCOMMODATION,  CONVERGENCE,  AND  TORSION. 
That  proposition  might  be  conceded  without  discussion.  But 
to  appreciate  its  importance,  especially  in  all  that  relates  to 
pathological  conditions,  it  is  necessary  to  go  over  the  ground 
carefully  step  by  step.  It  should  be  said  also  that  the  line 
of  study  will  not  always  be  easy.  Possibly  a  little  mental  gym- 
nastics may  be  required  occasionally,  and  although  we  shall 
follow  the  beaten  path  whenever  possible,  because  it  is  always 
the  safer  and  easier,  we  must  sometimes  pick  our  way  along 
where  the  footprints  of  other  students  are  few  and  indis- 
tinct. Short  digressions  will  also  occasionally  be  necessary, 
in  order  that,  as  we  proceed,  we  may  gather  physiological 
facts  which  are  near  at  hand  without  being  obliged  later  to 
retrace  our  steps.  The  value  of  these  facts,  although  not 
always  evident  at  the  time,  will  be  apparent  as  we  come 
to  the  pathological  aspects  of  the  subject.  Thus  advancing 
carefully,  if  we  reach  the  conclusion  just  referred  to,  with 
a  full  knowledge  of  all  that  it  means,  from  that  vantage 
ground  we  shall  see  many  of  the  pathological  questions 
relating  to  the  ocular  muscles  in  a  clearer  light  than 
would  be  possible  otherwise. 

§  2.  The  Geometry  of  the  Globe. — In  order  to  indi- 
cate definitely  the  position  which  the  eye  assumes  in  its 
various  movements  it  is  necessary  to  recall  certain  terms 
descriptive  of  the  globe  itself  and  certain  facts  concerning  it 
which,  though  familiar  to  most  ophthalmologists,  are  often 
forgotten  or  confused.  It  is  customary  to  consider  the  eye, 
like  the  earth,  as  divided  by  three  planes  at  right  angles 
to  each  other,  which  produce  corresponding  circles  by  their 
intersection  with  the  globe.  First,  there  is  a  horizontal 
plane  passing  through  the  center  of  the  cornea,  the  nodal 


I2O 


Principal  Planes  and  Axes 


points,  center  of  motion,  and  the  optic  nerve,  cutting  the 
globe  in  a  corresponding  horizontal  meridian.  Second,  a 
vertical  plane  passing  through  the  center  of  the  cornea,  the 
center  of  motion,  and  the  posterior  part  of  the  globe,  some- 
what externally  to  the  optic  nerve.  This  cuts  the  globe  in 
another  circle,  the  vertical  meridian.  The  intersections  of 
these  two  meridians  give  us  the  anterior  and  posterior  poles 
of  the  eye. 

Third,  an  imaginary  vertical  transverse  plane  passes 
through  the  center  of  motion,  cutting  the  globe  at  the 
equator,  and  is  therefore  called  the  equatorial  plane.  The 
equator,  with  the  horizontal  and  vertical  meridians,  are  natu- 
rally the  three  principal  or  great  circles  of  the  globe.  The 
edge  of  the  cornea  would  form  a  small  circle  if  it  were  actu- 
ally circular,  but  that  is  not  strictly  the  case. 

The  intersections  of  these  meridians  give  certain  well- 
known  axes  which  it  is  necessary  to  mention,  because  con- 
fusion exists  concerning  one  or  two  of  the  terms.  The 
vertical  axis  passes  through  the  two  points  where  the 
equator  intersects  the  vertical  meridian,  and  the  horizontal 
axis  through  the  two  points  where  the  equator  intersects 
the  horizontal  meridian.  The  axis  which  passes  through 


FIG.  Q3. — Listing's  Plane.  If  we  cut  a  circular  opening  in 
a  board  and  in  this  insert  a  rubber  ball,  passing  it  half  way 
through  the  opening,  the  board  represents  Listing's  plane. 

the  two  poles  of  the  eye  is  called  the  antero-posterior  or  the 
optic  axis  A  A  (Fig.  94).  Although  this  line  often  coin- 
cides with  the  center  of  the  cornea,  that  is  not  always  the 


Listing's  Plane 


121 


case.     In  a  subsequent  section  reference  will  be  made  to  the 
angle  which  the  visual  axis  makes  with  the  optic  axis. 

Listing's  Plane.— In  addition  to  those  geometrical  planes 
which  cut  the  globe  of  the  eye  as  similar  imaginary  planes 
cut  the  earth,  there  is  another  plane  which  we  shall  find 
of  importance  in  relation  to  movements  of  the  globe. 
It  is  the  so-called  Listing's  Plane  (Fig.  93).  This  is  a 
vertical  transverse  plane  passing  through  the  center  of  wot  ion 
of  both  eyes.  It  is  always  considered  as  fixed  and  immov- 
able. It  coincides  with  the  equatorial  plane  of  the  eyes  only 
when  the  visual  axes  are  in  the  primary  position — that  is, 
when  they  lie  in  the  horizontal  plane,  are  parallel  to  each 
other,  and  are  perpendicular  to  the  base  line  connecting 
the  center  of  motion  of  the  two  eyes  (B  264,  p.  289).  The 
importance  of  Listing's  Plane  will  become  evident  when  we 


FIG.  94. — Horizontal  section,  partly  schematic,  through  the  two 
orbits.  A  A  antero-posterior  axis,  T  T  transverse  axis,  D  D  axis  of 
rotation  of  the  superior  and  inferior  recti,  O  O  axis  of  rotation  of  the 
oblique  (Landolt). 

see  later  that  every  turn  of  the  globe  from  the  primary  to 
a  secondary  position  is  accomplished  by  a  rotation  about 
an  axis  which  lies  in  this  plane. 

It  is  well  also  to  recall  the  position  of  the  axes  about 


122  Axes  of  the  Recti 

which  the  globe  rotates  when  it  is  acted  on  by  certain 
muscles  or  groups  of  muscles. 

First,  as  to  the  axis  of  the  horizontal  muscles.  As  the 
center  of  the  insertion  of  the  internal  rectus  corresponds  gen- 
erally to  the  horizontal  plane,  and  as  the  center  of  the 
external  rectus  also  corresponds  to  the  horizontal  plane, 
it  is  evident  that  the  action  of  either  of  these  muscles  is 
to  rotate  the  eye  about  an  axis  which  is  practically  vertical. 

Second,  the  axis  of  the  vertical  muscles  does  not  coincide 
with  the  horizontal  axis  of  the  eye.  For,  as  the  superior 
and  inferior  recti,  arising  near  the  median  line,  pass  forward 
and  outward,  they  are  inserted  into  the  globe  at  an  angle  of 
about  twenty-five  degrees  from  the  median  plane.  This  angle 
is  not  really  invariable,  as  the  positive  statements  in 
some  of  the  text-books  would  lead  one  to  expect,  for,  as  we 
have  seen,  the  primary  and  secondary  insertions  of  these 
muscles  vary  very  decidedly.  It  is  evident  that  the  axis  of 
rotation  of  these  muscles  forms  a  corresponding  angle 
with  the  optic  axis  on  the  horizontal  plane.  When  therefore 
the  superior  rectus,  for  example,  contracts,  it  does  not  turn 
the  globe  directly  up,  but  up  and  outward  (Fig.  94). 

Third,  as  to  the  axis  of  the  oblique  muscles.  In  a  similar 
manner,  the  axis  of  rotation  of  the  oblique  muscles  is  usually 
described  as  making  an  angle  of  about  thirty-five  degrees 
with  the  optic  axis,  but  for  the  same  anatomical  reasons 
this  statement  is  only  approximately  true.  This  axis,  like 
that  of  the  superior  and  inferior  recti,  of  course  does  not  lie  in 
Listing's  plane,  and  when  the  superior  oblique,  for  example, 
contracts,  it  does  not  rotate  the  globe  down,  but  down  and 
outward. 

It  is  worth  while  also  to  glance  at  a  table  giving  the 
average  measurements  of  the  globe  and  of  the  distances 
within  it. 

According  to  Helmholtz  we  have  the 

Length  of  the  emmetropic  eye 23.266  mm 

Radius  of  curvature  of  the  cornea 7.829  " 

Distance  from  the  apex  of  the  cornea  to  the 

anterior  surface  of  the  crystalline  lens 3.6  " 


The  Center  of  Motion  123 

Radius  of  curvature  of  the  anterior  surface  of 

the  lens 10.0       mm. 

Radius  of  curvature  of  the  posterior  surface. .      6.0          " 

Thickness  of  the  lens 3.6          " 

Distance  from  the  apex  of  the  cornea  to  the 

posterior  surface  of  the  crystalline  lens 7.2          " 

§  3.  The  Center  of  Motion. — We  know  that  the  action 
exerted  by  any  muscle  is  in  a  plane  determined  by  three 
points:  the  origin  of  the  muscle,  its  insertion,  and  the  center 
of  motion  of  the  eye.  The  origins  and  insertions  of  the 
different  muscles  have  been  already  studied.  A  number  of 
different  methods  have  been  suggested  by  which  the  center 
of  motion  may  be  determined,  and  the  results  obtained  vary 
slightly,  but  perhaps  the  simplest  and  most  reliable  is  that 
proposed  by  Donders  and  Dojer  (B  251).  This  article 
was  evidently  considered  of  special  value  by  Donders, 
for  a  large  part  of  it  is  reproduced  almost  verbatim  in 
his  work  on  Accommodation  and  Refraction  (B  260,  p. 
1 86).  The  method  consists  in  determining  the  diameter 


FKJ.  95.— Doubling  of  the  FIG.  96.— Triangles  in 

corneal    images   as  seen  with  the   globe    by   which   the 

the      ophthalmometer     when  center  of  motion  is  meas- 

measuring     the      center      of  ured  (Donders). 
motion  (Donders). 

of  the  cornea,  or  its  half  diameter,  by  means  of  an 
ophthalmometer,  and  then  estimating  from  that  diameter 
where  the  center  of  motion  lies  (Figs.  95  and  96). 

By  this  method  it  was  found  that  the  distance  of  the 
center  of  motion  behind  the  anterior  surface  of  the  cornea 
was 

In  emmetropia 13-45  mm- 

In  hypermetropia  (about) 13.22     " 

In  myopia  (about) H-52     " 


124         Calculation  of  the  Center  of  Motion 

§  4.     Calculation  of  the  Center  of  Motion  by  Means 
of  the  Javal-Schiotz  Ophthalmometer. 

It  might  be  sufficient,  after  referring  briefly  thus  to  the  method  adopted  and 
the  results  obtained,  to  leave  the  subject  here.  But  some  students  may  care  to 
follow  it  a  step  further,  making  the  measurements  themselves,  and  for  that 
reason  an  additional  word  is  in  order,  to  indicate  how  the  center  of  motion 
can  be  determined  by  means  of  the  Javal-Schiotz  ophthalmometer  now  so 
commonly  used,  at  least  in  America. 

For  this  purpose,  it  is  necessary  to  place  the  instrument  on  a  table  about 
a  meter  and  a  half  or  three-quarters  in  length,  and  draw  the  telescope  back- 
ward from  the  head-rest  until  the  distance  between  them  measures  1.33  meters. 
Of  course  the  mires  are  dispensed  with  altogether.  A  small  electric  light 
or  a  candle  is  placed  directly  above  or  below  the  tube,  but  in  order  to  focus 
the  cornea,  the  optical  arrangement  requires  that  the  tube  should  be  propor- 
tionately shortened.  One  conjugate  distance — from  patient  to  instrument — 
having  been  lengthened,  the  other  conjugate  distance — from  instrument  to 
observer — must  be  shortened. 

The  easiest  way  to  accomplish  this  is  to  unscrew  the  tube  at  the  point 
where  the  larger  portion  of  it  joins  with  the  smaller — that  is,  take  out  what 
may  be  called  the  slim  joint. 

It  is  necessary,  therefore,  to  place  a  proper  eye-piece  at  the  end,  in  order  to 
see  the  double  image  of  the  cornea  distinctly.  For  this  purpose  a  twenty- 
diopter  glass,  or  the  ordinary  eye-piece  of  the  ophthalmometer,  can  be  used, 
by  having  a  collar  fitted  to  it  of  sufficient  size  to  adapt  it  to  the  larger  part  of 
the  tube.  Such  an  arrangement  gives  a  clear  double  image  of  the  cornea,  or 
by  changing  the  distance  of  the  instrument  from  the  patient  the  two  circles 
of  the  cornea  can  be  made  to  overlap  each  other  to  any  extent  desired. 

The  second  modification  of  the  ophthalmometer  necessary  for  this  purpose 
is  to  attach  to  it  a  bar  on  which  to  record  the  amount  which  the  globe  turns 
from  one  side  to  the  other.  For  this,  we  place  a  small  brass  bar,  which  meas- 
ures about  seventy  centimeters  long  by  five  or  six  millimeters  square,  horizon- 
tally across  the  center  of  the  instrument.  It  happens  that  the  earlier 
forms  of  the  Javal-Schiotz  ophthalmometer  have  a  horizontal  slit  in  the  disc, 
and  it  is  therefore  easy  to  attach  the  bar  by  means  of  a  couple  of  thumb- 
screws. The  rod  should  be  graduated  in  centimeters  from  its  central  point 
outward  in  each  direction  for  a  distance  of  about  twenty  centimeters,  and  from 
that  point,  to  its  end,  in  millimeters. 

On  this  bar  are  two  small  carriers  which  slide  on  the  bar  at  will 
and  serve  as  objects  at  which  the  patient  looks.  In  order  to  make  these 
more  distinct,  it  is  sometimes  desirable  to  attach  to  each  one  a  bit  of  paper  or 
other  object  easily  distinguishable. 

The  third  change  in  the  ophthalmometer  is  to  attach  to  the  part  against 
which  the  head  rests,  a  ring  or  square  which  has  a  hair  strung  vertically  across  itt 
and  so  arranged  that  it  can  be  brought  close  to  the  cornea  under  examination. 
The  vertical  hair  in  the  ring  should  also  be  so  near  to  the  cornea  that  both 
hair  and  cornea  are  focused  at  the  same  time,  and  so  adjusted  by  a  slide  that 
the  ring  can  be  moved  slightly  from  side  to  side  until  it  is  just  opposite  the 


Calculation  of  the  Center  of  Motion         125 

center  or  edge  of  the  cornea.     Having  made  these  three  alterations  the  pro- 
cedure is  simple. 

First,  we  must  measure  the  diameter  of  the  cornea.  To  do  this  the  distance 
between  the  instrument  and  the  observed  eye  is  increased  until  the  doubling  of 
the  image  is  such  that  the  edge  of  the  one  cornea  passes  through  the  center 
of  the  image  of  the  other  cornea  (Fig.  95).  The  distance  from  the  center 
of  the  prisms  to  the  center  of  the  cornea  is  then  measured.  This  distance 
D  E  we  will  call  a  (see  Fig.  97). 


FlG.  97. — Triangle  on  which  calculations  are  based  to  determine  the  center 
of  motion  of  the  globe. 


Half  of  the  breadth  of  the  cornea  is  found  in  the  following  manner: 
measurements  show  that  the  ophthalmometer  used  is  so  constructed  that 
when  an  object  is  placed  330  mm.  from  the  center  of  the  prism  the  in- 
strument produces  a  deviation  of  2.95  mm.  of  the  image  of  the  object. 
Hence  if  the  cornea  is  placed  at  a  distance,  a,  and  is  deflected  a  distance 
equal  to  its  own  half  breadth,  X,  we  have  the  following  proportion  : 


and  x= 


330:  2.95 
2.95 


330 


(I) 


In  order  to  measure  the  center  of  motion  the  patient  is  requested  to  fix  his 
eyes  on  one  of  the  small  carriers  of  the  bar,  which  is  placed  directly  over  the 
center  of  the  instrument ;  the  hair  is  then  adjusted  until,  on  looking  through 
the  ophthalmometer,  it  seems  to  pass  exactly  through  the  center  of  the  cornea. 
The  carrier  which  the  person  is  still  observing  is  then  moved  along  the  bar, 
until  it  reaches  such  a  position  that  as  the  examiner  looks  through  the  ophthal- 
mometer the  hair  seems  just  to  touch  the  edge  of  the  observed  cornea.  The  dis- 
tance from  the  carrier  to  the  center  of  the  rod — DA  or  DB —  is  then  measured, 
which  distance  we  shall  afterwards  refer  to  as  b  (see  Fig.  97).  It  is  wise  in 
this  connection  to  perform  the  same  operation  on  the  other  half  of  the  bar  in 


126 


Calculation  of  the  Center  of  Motion 


order  to  check  the  result.  If  the  observation  has  been  made  carefully  the  two 
distances  should  be  equal.  We  are  now  ready  to  calculate  the  center  of 
motion. 

FIG.  97. — Let  ADB  be  the  horizontal  rod,  A  and  B  the  carriers,  PEG  the 
chord  of  the  cornea,  C  the  center  of  motion.  The  eye  is  so  placed  that  FG  is 
parallel  to  AB  and  the  line  CD  passes  through  C  the  center  of  motion,  E  the 
center  of  the  chord,  and  D  the  center  of  the  telescope.  Hence  let  DE  =  a 
EC  =y,  AD  =  b.  From  expression  (i),  we  have  330:  2.95::  a:  FE. 

FE  =         ' — Let  — =  m,   which  is  a  constant  for  any  given    instrument, 

330  330 

then  FE  =  ma.     In  the  similar  triangles  FEC  and  ADC,  DC  :  AD  :  :  EC  :  FE. 

y  +  a  :  b  :  :  y  :  ma.      mya  +  ma*  =  yb.      ma?=yb  —  mya  =y  (b  —  ma),    y  = 

ma* 
b — ma 

If  we  call  m  "the  deviating  power  of  the  prism,"  then  we  can  express  this 
formula  as  a  rule  which  in  reality  is  not  as  complicated  as  it  sounds. 

The  distance  of  the  center  of  motion  from  the  transverse  chord  of  the  cornea 
is  equal  to  the  quotient  of  the  product  of  the  deviating  power  of  the  prism 
multiplied  by  the  square  of  the  distance  from  the  transverse  chord  of  the 
cornea  to  the  brass  rod,  divided  by  the  difference  between  the  distance  of  the 
pointers  from  the  center,  and  the  product  of  the  deviating  power  of  the  prism 
multiplied  by  the  distance  from  the  brass  rod  to  the  cornea. 

The  following  is  a  calculation  of  this  rule  arranged  for  logarithmic  com- 
putation. 


m  = 


2-95 
330. 


a  =  662.5,  b  =  359.3. 
log.     2.95         = 
log.     330.         = 


m  = .008938 

log.  m 
log.    a 

log.  ma 


.469822-1-0. 
.5 18514+2. 
.951308—3  =log.  .008938 


.951408—3 
.821186+2 

.772494+0 

.821186+2 

.593680+0 
.548242+2 


b  = 


=  log. 


359-3 


5.9223 


353.38  =  (b  —ma) 


log.  ma5     = 
log.  .(  b— ma  )     : 

.045438+1 

Add  the  distance  of  the  chord  of  the 
cornea  from  its  anterior  surface  or 
the  vers.  sin  of  corneal  arc 

Location  of  center  of  motion  behind 
the  cornea  (radius) 


=  log.         11.1029 


2.5 


13.60+ 


§  5.  The  Angle  Alpha. — The  text-books  usually  rep- 
resent the  lens  and  cornea  as  if  both  were  exactly  cen- 
tered. We  have  already  seen  that  the  former  is  often 


Angles  Alpha,  Delta,  and  Gamma.  127 

displaced  from  the  position  which  theoretically  it  should 
occupy,  and  exact  measurements  also  show  that  the  center 
of  the  curvature  of  the  cornea  docs  not  always  correspond 


FIG.  98. — Horizontal  section  of  the  globe  with 
distortion  of  natural  curvature  of  the  cornea  to 
show  the  direction  of  the  various  axes  in  the  hori- 
zontal plane  ( Landolt). 

to  the  axis  of  the  lens.  All  have  agreed,  however,  to  call 
the  optic  axis  the  line  which  passes  through  the  nodal 
points  of  the  lens  and  approximately  the  center  of  the 


128 


Angle  Alpha  in  E.  H.  and  M. 


cornea  (A  A,  Fig.  98),  and  also  to  call  the  -visual  axis  the 
line  which  passes  from  the  object  O  to  the  fovea,  OF. 
Unfortunately  confusion  has  arisen  as  to  the  name  of  the 
angle  which  the  optic  axis  makes  with  the  visual  axis,  and  it 
is  well  to  clear  up  this  point  before  proceeding  farther. 
When  Bonders  made  his  important  investigations  concern- 
ing the  size  and  position  of  what  he  called  the  "angle 


FIG.  99. — The    angle    alpha  in   emmetropia,  myopia,   and  hypermetropia. 
ag  is  the  optic  axis  ;  //',  the  visual  axis  (Bonders). 


alpha,"  he  referred  to  the  one  which  the  visual  axis  makes 
with  the  optic  axis.  Since  his  time,  however,  it  was  found 
that  the  apex  of  the  corneal  ellipse  (E)  does,  not  always 
coincide  with  a  point  in  the  optic  axis  (C).  Hence  OXE 
was  called  by  Landolt  and  some  others  the  angle  alpha 
and  OMA  the  angle  gamma.  But  many  other  writers  still 
describe  the  angle  alpha  as  the  one  which  the  optic  axis 
makes  with  the  visual  axis,  that  is  OXA.  Maddox  (B  263. 
p.  217)  tries  to  clear  up  the  confusion  by  describing  the 
"  angle  alpha  of  Bonders  "  as  one  angle,  and  the  "  angle  alpha 
of  Landolt "  as  another.  In  order  to  be  rid  of  this  ambi- 
guity, it  seems  better  to  follow  the  example  of  Bonders,  as 
Tscherning  and  others  have,  and  retain  the  term  "  angle 


Clinical  Value  of  Angle  Alpha  1 29 

alpha  "  to  describe  the  one  which  the  visual  axis  makes  with 
the  optic  axis  OXA,  to  agree  with  Landolt  in  calling 
the  angle  OMA  the  angle  gamma,  and  then  to  the  angle  OXE 
give  an  entirely  different  name — for  example,  the  angle 
delta. 

It  happens  that  we  have  to  deal  frequently  with  this 
angle  alpha,  but  the  angles  delta  and  gamma  are  only  of 
theoretical  importance. 

The  size  of  the  angle  alpha  varies.  In  emmetropia  it 
ranges  ordinarily  from  three  to  five  or  six  degrees  or  some- 
times much  more.  When  unusually  large,  the  eye  has  every 
appearance  of  a  divergent  squint,  although  the  visual  axes 
may  be  perfectly  parallel.  In  myopia  the  angle  is  less  than 
in  emmetropia,  in  fact  it  is  often  reduced  to  nothing,  and 
sometimes  the  anterior  end  of  the  visual  axis  falls  to  the 
temporal  side  of  the  optic  axis.  In  that  case  the  angle  is  said 
to  be  negative  (Fig.  99). 

§  6.  Clinical  Value  of  the  Angle  Alpha.— Of  what 
importance  is  the  angle  alpha  and  why  is  it  worth  while  to 
consider  methods  for  its  measurements  ? 

First.  Two  of  the  methods  of  measuring  this  can  also  be 
made  use  of,  with  slight  modifications,  to  measure  patho- 
logical deviations  of  the  eyes,  and  if  given  here  they  need 
not  be  described  later. 

Second.  The  supposed  divergence  of  some  hyperme- 
tropes  can  be  shown  to  be  only  apparent. 

Third.  A  large  angle  alpha  may  act  as  a  predisposing 
cause  of  pathological  deviations. 

There  are  several  methods  by  which  these  measurements 
can  be  made :  one  is  simple,  but  only  an  approximate  esti- 
mate ;  others  are  more  exact,  but  demand  time  and  care. 

(a)  The  easiest  method  is  to  estimate  the  size  of  the 
angle  from  the  apparent  position  of  the  corneal  reflex  with 
reference  to  the  center  of  the  pupil  (B  263,  p.  216).  For 
that  purpose  the  ophthalmoscope  is  sufficient.  When  the 
observed  eye  looks  straight  at  the  opening  in  the  center 
of  the  mirror,'  the  visual  axis,  of  course,  passes  through 
the  inner  side  of  the  observed  cornea  to  the  fovea  (Fig. 
100).  If  now  the  angle  be  zero  or  very  small,  the  reflex 


130  Measurements  of  the  Angle  Alpha 

from  the  cornea  appears  in  the  center  of  the  pupil.  If 
the  angle  be  large,  the  reflex  from  the  cornea  seems  to  be 
toward  the  inner  edge  of  the  pupil.  If,  as  may  happen  in 
myopia,  the  angle  be  negative,  then  the  corneal  reflex  will 
be  seen  nearer  the  outer  edge  of  the  pupil. 

In  such  measurements  one  must  be  certain,  however,  that 
the  pupil  of  the  observed  eye  is  central  and  normal,  and 
also  that  the  person  looks  at  the  opening  in  the  ophthalmo- 
scope. The  simplicity  of  this  method  gives  it  great  value, 
not  only  for  estimates  of  the  angle  alpha,  but  also  in  detect- 
ing the  degree  and  forms  of  deviations,  though  it  must  be 
admitted  that  these  measurements'  are  not  exact. 


FIG.  100. — Ophthalmoscopic  corneal  reflections  in  emme- 
tropic  eyes  :  above  with  both  eyes  looking  at  the  center  of 
the  mirror ;  below  with  both  eyes  looking  to  the  right, 
showing  a  symmetry  of  the  corneal  images  owing  to  the  angle 
alpha  (Maddox). 

(b)  Another  simple  method  of  measuring  this  angle, 
which  is  also  dependent  on  the  corneal  reflex,  is  by  means 
of  the  perimeter,  with  an  electric  light  or  candle  which 
moves  along  the  arc  when  that  is  placed  horizontally.  Let 
us  suppose  the  right  eye  to  be  under  examination.  The 
head  having  been  adjusted  in  the  usual  way  before  the 
instrument,  the  patient  is  directed  first  to  look  at  the  zero 
point  of  the  arc.  If  at  the  same  time  the  light  be  placed  at 
the  zero  point,  and  the  examiner,  sitting  in  front,  sights 
over  this  point  into  the  eye  of  the  patient,  the  corneal  reflex 
seems  to  come  from  the  inner  portion  of  the  pupil.  If, 
however,  the  examiner  continues  to  sight  over  the  zero 


Types  for  Testing  the  Accommodation         131 

point  while  the  light  is  slid  along  the  arc  to  the  left  of  the 
patient,  and  his  eye  then  follows  the  light,  a  point  is  soon 
reached  at  which  the  examiner  sees  the  corneal  reflex  in  the 
center  of  the  pupil.  The  number  of  degrees  traversed  by 
the  light  is  the  size  of  the  angle  alpha. 

(c)  A  more  exact  method  of  measuring  the  angle  is  to 
compare  the  position  of  the  reflex  from  the  cornea  with  that 
which  comes  from  the  posterior  capsule  of  the  lens.  This 
has  been  already  described  when  considering  the  position  of 
the  lens  (page  71).  When  exactness  is  desired,  this 
method  is  certainly  the  best.  For  this  purpose  the  ophthal- 
mophacometer  of  Tscherning  is  not  necessary,  the  ophthal- 
mometer  of  Javal  with  the  modifications  already  described 
being  quite  sufficient. 

§  7.  The  Relation  of  Visual  Acuity  to  the  Action 
of  the  Eye. — In  connection  with  the  geometry  of  the  globe, 
we  may  with  propriety  consider  that  which  determines  the 
direction  of  the  eye  when  in  motion — in  other  words,  the 
point  of  fixation  of  the  eye  and  the  acuity  of  vision. 
The  position  which  the  globe  assumes  normally  is  deter- 
mined by  the  fact  that  the  sensibility  at  the  fovea  is  so 
much  greater  than  elsewhere  in  the  retina,  that  there  is  an 
instinctive  desire  to  turn  the  eye  in  such  a  way  that  the 
central  part  of  the  image  shall  fall  just  at  that  point. 
Exact  studies  made  by  Uhthoff  and  others  indicate  that  the 
smallest  space  between  two  points  which  can  be  perceived 
must  subtend  an  angle  of  about  55  seconds. 

This  fact  has  a  bearing  upon  the  construction  of  test 
types,  for,  as  is  well  known,  most  of  the  letters,  especially 
the  square  ones,  can  themselves  be  resolved  into  squares, 
the  smallest  projecting  parts  measuring  about  one-fifth  of  the 
entire  letter.  Therefore,  in  order  to  see  all  the  parts  of 
such  a  letter  distinctly,  each  of  these  smallest  portions  must 
subtend  an  angle  of  about  one  minute,  or — exactly — the 
entire  letter  should  subtend  an  angle  of^  55"  X  5  =  4°.6. 
Knowing  this,  it  is  easy  by  simple  trigonometry  to  ascer- 
tain what  the  total  height  of  a  letter  should  be  in  order  to 
have  its  smallest  portions  visible  at  a  given  distance.  If, 


132         Types  for  Testing  the  Accommodation 

for  example,  we  wish  to  construct  a  letter  just  visible  at 
one  hundred  meters,  we  have  100  tan.4°.6  =  x  =  133.0  mm. 
In  like  manner  we  find  that  the  height  of  a  test  letter  for  a 
distance  of  50  meters  should  be  66.5  millimeters;  for  25 
meters,  33.3  millimeters,  etc. 

It  is  necessary  thus  to  call  attention  briefly  to  the 
well  known  principle  upon  which  the  construction  of 
proper  test  types  depends,  because,  when  measuring  the 
action  of  the  ciliary  muscle,  physiologically  or  patho- 
logically, we  must  know  that  the  object  looked  at  is 
of  the  size  to  be  seen  readily  by  the  normal  eye 
at  a  given  distance.  As  far  as  the  types  are  concerned 
which  are  used  for  distance,  it  makes  comparatively  little 
difference  which  set  is  selected  out  of  the  several  ex- 


E 


10 


6 

5MF 

4H  S 

3  N  T  L 

2     OHVE 

|5  DX  ZCE 

Y    L    E     O     K 

0.75         E      X       P      H       8 

0.50     •    •    «  .  «    o 
0.33 

FIG.  101. — Series  of 
test  types  for  the  near 
point. 


Suppression  of  Diplopia  133 

cellent  forms  which  have  been  constructed  on  this  plan 
by  ophthalmologists.  When  making  tests  at  the  near 
point  it  is  desirable,  however,  to  use  types  which  permit 
the  vision  to  be  expressed  in  meters  or  fractions  of  a 
meter,  if  we  are  to  make  this  part  of  our  ophthalmology 
accord  with  the  rest.  Among  all  the  test  types  ordinarily 
used  for  the  near  point  it  was  not  easy  to  find  any  which 
were  entirely  satisfactory,  and  accordingly  another  set  was 
arranged  (Fig.  101).  They  seem  to  have  some  advantages. 

First,  they  are  constructed  on  the  metric  system  and  bear 
a  definite  and  convenient  relation  to  each  other.  They  re 
also  in  accord  with  the  letters  recommended  by  the  Com- 
mittee of  the  American  Ophthalmological  Society  (B  266). 

Second,  the  largest  is  of  such  a  size  that  it  should  be  seen 
by  the  normal  eye  at  ten  meters,  the  next  at  six,  then  five, 
four  meters,  and  down  to  one-third  of  a  meter,  the 
distance  being  clearly  expressed  in  the  margin  of  the 
card.  When  using  these  for  the  near  point  (p),  that  can 
easily  be  recorded, — p  =  0.5  or  p  =  0.75,  etc.,  as  the  case 
may  be. 

Third,  the  detached  letters  here  used  are  better  than 
words. 

Fourth,  the  tests  as  a  whole  are  so  small  that  when  not 
framed  they  can  be  carried  in  the  pocket,  and  are  always 
ready  for  use. 

Fifth,  the  card  on  which  the  letters  are  printed  has 
attached  to  it,  when  framed,  a  thread  with  several  knots,  one 
at  33  cm.  another  at  50  cm.,  etc.,  so  that  the  exact  distance 
at  which  the  types  are  held  can  thus  be  measured  easily 
and  promptly. 

§  8.  Suppression  of  Diplopia  is  Physiological. — When 
both  eyes  are  fixed  upon  an  object  in  front,  evidently  all 
other  objects  lying  in  that  plane — or  indeed  anywhere  else 
except  in  the  circle  of  the  horopter — are  focused  on  parts 
of  the  retina  in  the  two  eyes  which  do  not  correspond  with 
each  other.  This  of  course  produces  double  vision,  and  if 
we  were  accustomed  to  take  cognizance  of  all  these  double 
images  the  result  would  be  confusing  in  the  extreme.  That 
is  easily  seen  by  pressing  upon  one  eye  in  any  direction,  so 


134  Monocular  Position  of  Rest 

as  to  produce  diplopia.  The  fact  is,  therefore,  that  the 
normal  eye  is  accustomed  to  suppress  those  images  which 
do  not  fall  on  the  fovea,  and  to  such  an  extent  that  we  are 
practically  unconscious  of  it.  This  is  accounted  for  in 
various  ways,  and  perhaps  no  better  explanation  has  been 
given  than  by  what  Javal  calls  "  the  antagonism  of  the  visual 
fields.  "  Or,  as  Tscherning  says  (B  264)  :  "  It  is  sometimes 
the  images  of  one  eye  that  predominate,  sometimes  those  of 
the  other,  and  as  long  as  we  see  in  a  part  of  the  visual  field 
images  with  one  eye,  those  of  the  other  eye  are  completely 
suppressed."  Javal  considers  that  this  has  an  important 
bearing  on  some  forms  of  deviation  with  which  we  will 
have  to  deal  later. 

§  9.  Monocular  Position  of  Rest.— It  is  customary  to 
suppose  that  when  one  eye  is  at  rest  it  is  in  the  primary 
position,  but  that  is  not  always  the  case.  The  observations 
which  we  have  on  this  point  rather  indicate  that  the  tend- 
ency of  a  single  eye,  when  at  rest,  is  to  swing  from  the 
primary  position  sometimes  inward,  or  more  frequently  out- 
ward, or  outward  and  upward.  Certain  experiments  also 
indicate  that  when  a  single  eye  fixes  an  object  and  the  light 
is  suddenly  extinguished,  or  the  object  which  is  looked  at 
vanishes,  the  globe  turns  slowly  outward  a  few  degrees. 
Maddox  (284)  has  studied  this  phenomenon  quite  carefully 
with  what  he  calls  a  visual  camera.  On  trying  some  of  the 
experiments  with  this  camera  I  have  found  them  inter- 
esting, though  not  apparently  of  clinical  value. 

He  thinks  that  when  an  individual  excludes  one  eye 
from  the  visual  act,  the  other  apparently  tends  to 
swing  outward  rather  more  frequently  and  to  a  greater 
degree  than  inward.  This  statement  apparently  contradicts 
what  we  find  constantly  in  practice — namely,  a  slight  degree 
of  latent  convergence,  or  esophoria.  The  position  which 
the  eye  assumes  in  sleep  also  indicates  that,  as  a  rule,  it 
turns  up  and  outward.  It  is  frequently  stated  that  eyes 
which  have  become  blind  almost  invariably  turn  outward. 
That,  however,  is  not  quite  true,  for  an  examination  of  one 
hundred  and  twenty-one  pupils  of  the  New  York  State 
School  for  the  Blind  with  reference  to  this  point  showed 


Monocular  Position  of  Rest  135 

that  there  were  almost  as  many  cases  of  abnormal  conver- 
gence as  of  divergence,  though  sometimes  the  amount  of 
nystagmus  or  the  degree  of  distortion  of  the  globe  made  it 
rather  difficult  to  decide  just  what  position  a  given  eye  did 
assume.  We  must  conclude,  therefore,  that  the  monocular 
position  of  rest  seldom  corresponds  to  the  primary  position. 


CHAPTER  II. 

§  i.  One  Eye  in  Action  but  not  Necessarily  in 
Motion  (Accommodation). — Earlier  students  supposed 
that  accommodation  was  produced  by  elongation  of  the 
globe,  advancement  of  the  lens,  contraction  of  the  pupil, 
increase  in  curvature  of  the  cornea,  etc.  Without  delaying 
to  consider  how  this  act  is  not  accomplished,  let  us  see  briefly 
in  what  it  does  consist.  We  all  agree  now  that  this  is  by  a 
contraction  of  the  ciliary  muscle,  and  as  a  result,  the  lens  in 
some  way  becomes  more  convex  (Fig.  102).  It  is  still  a 
question,  to  some,  however,  whether  the  zonula  is  relaxed,  or 
whether  it  is  tense  in  extreme  accommodation,  and  also  as 
to  the  exact  form  which  the  lens  assumes  when  increasing  its 
convexity.  There  are  two  opposing  views  concerning  the 
condition  of  the  zonula.  The  first  of  these,  and  the  one 
accepted  at  present  by  most  physiologists,  is  the  so-called 
Helmholtz  theory  of  accommodation.  That  may  be  stated 
briefly  as  follows : 

When  the  individual  looks  at  a  far  point,  the  ciliary 
muscle  is  reiaxed,  but  in  such  a  manner  that  the  fibers  of  the 
zonula  are  held  tense,  and  this  traction  on  the  lens  causes  it 
to  become  thin  and  adapted  to  focusing  parallel  rays 
upon  the  retina.  But,  when  the  individual  looks  at  a  near 
point,  the  contraction  of  the  ciliary  muscle  draws  the  entire 
ring  of  muscle  toward  the  lens,  producing  a  relaxation  of  the 
zonula.  The  ultimate  fibers  of  the  lens,  being  then  relieved 
from  pressure,  tend  to  straighten  themselves  out  and  make 
the  lens  more  convex.  Or,  according  to  this  view,  the 
zonula  is  tense  when  the  eye  is  adjusted  for  the  distance, 
and  it  relaxes  more  and  more  in  proportion  to  the  degree  of 
accommodation  for  a  near  point. 

On  the  other  hand,  according  to  the  view  which  has  been 

136 


The  Act  of  Accommodation 


137 


more  recently  elaborated  by  Tscherning,  it  is  considered 
that  when  the  eye  is  adjusted  for  distance  it  is  entirely 
at  rest.  The  ciliary  muscle  is  relaxed,  the  zonula  is  also 
relaxed,  and  the  convexity  of  the  normal  lens  is  then  just 
sufficient  to  produce  a  clear  image  upon  the  retina  of  the 
normal  eye.  When,  however,  an  effort  is  made  at  accom- 
modation, the  contraction  of  the  ciliary  muscle  produces  a 
tension  of  the  zonula,  and  this,  of  such  a  character  as  to  cause 
an  increase  in  the  convexity  of  the  anterior  surface  of  the  lens, 
or,  as  Tscherning  calls  it,  "  a  temporary  anterior  lenticonus." 
All  agree,  therefore,  that  accommodation  is  due  to  an  effort  on 
the  part  of  the  ciliary  muscle.  For  our  purposes  it  might  be 
sufficient  to  state  the  important  fact  that  accommodation  is 
essentially  an  active  condition.  But  as  it  is  necessary  to 


FIG.  102. — Change  in  the  eye  during  accommodation  (Helmholtz). 

make  frequent  reference  to  accommodation  in  all  of  the 
physiological  and  especially  in  the  pathological  part  of  these 
studies,  it  is  therefore  desirable  at  this  point  to  review  briefly 
the  different  factors  which  enter  into  that  act  in  order  that 
there  may  be  no  question  as  to  what  is  meant  by  the  term 
"accommodation."  Let  us,  therefore,  consider  the  phe- 
nomena which  take  place. 

.First.  The  pupil,  which  is  dilated  when  viewing  distant 
objects,  becomes  contracted  when  the  eye  is  adjusted  for  a 
near  point.  Of  this  there  is  no  question. 

Second.  The  pupillary  edge  of  the  iris  apparently  changes 
its  position.  Helmholtz  thinks  that  it  advances.  Tscherning 
(B  303)  considers  that  more  apparent  than  real,  and  figures 


1 38  How  is  the  Zonula  Affected  ? 

a  section  of  the  anterior  chamber  as  in  the  accompanying 
diagram  (Fig.  103). 

Third.  The  ciliary  muscle  contracts  and  this  contraction 
is  generally  in  proportion  to  the  degree  of  accommodation. 
This  is  also  agreed  to  by  all,  the  proofs  being : 

(A)  The  subjective  sensation. 

(B)  The  phenomena  produced  by  a  cycloplegic. 

(C)  The  phenomena  produced  by  paralysis  of  the  motor 
oculi. 

Fourth.  The  effect  which  that  contraction  of  the  ciliary 
muscle  has  upon  the  zonula  is  not  yet  fully  understood. 


FIG.  103. — Changes  of  the  anterior  chamber  during 
accommodation.  a  Repose.  b  Accommodation. 
(Tscherning.) 

Several  men  who  have  studied  the  question  most  carefully  and 
exactly  still  differ  among  themselves  as  to  whether  the 
zonula  is  then  relaxed  or  is  made  more  tense. 

In  favor  of  the  former  view  we  have  the  following  contentions  : 

(A)  The  entire  lens  seems  to  fall  toward  the  more  dependent  portion  of  the 
eye.     Hess  (B  327)  and  Heine  (B  314)  consider  this  as  fully  established,  while 
Tscherning  accounts  for  the  appearance  by  a  change  of  the  position  of  the 
entoptic  images.     It  is  a  point  not  easy  to  decide. 

(B)  Under  favorable  circumstances,  when  accommodation  begins,  the  entire 
lens  can  be  seen  to  shake  or  tremble  with  motions  of  the  eyes  or  head.  Although 
Hess  was  the  first  one  to  call  attention  to  the  meaning  of  this  phenomenon, 
it  is  so  easily  observed  that  it  is  surprising  its  significance  had  not  been  recog- 
nized before.     It  is  demonstrated  best  in  some  case  in  which  there  is  a  small 
but  well-defined  opacity   of  the   lens   near  its   center,   such   as  we  see,  for 
example,  in  certain  cases  of  injury  or  in  forms  of  well-marked  lamellar  cata- 
ract.    If  we  first  drop  into  such  an  eye  a  moderately  strong  solution  of  cocain, 
the  dilatation  of  the  pupil  enables  the  opacity  and  a  considerable  portion  of  the 
lens  to  become  visible.    Then  if  a  solution  of  eserin  be  applied  and  after  a  few 
minutes  the  lens  be  examined  by  oblique  illumination  or  with  the  ophthalmo- 
scope, or  even  with  the  naked  eye,  it  is  possible  to  observe  that  each  time  the 
globe  makes  a  sudden  motion  as  the  patient  looks  to  the  right,  left,  up,  or 
down,  the   cataractous   opacity,  and  the  entire   lens  with  it,  can  be  seen  to 
shake  and  tremble.     The  appearance  presented  under  such  circumstances  is 
very  similar  to  that  which  is  observed  when  for  any  reason  the  vitreous  has 
become  fluid.     In  this  experiment  it  should  be  noticed  that  when  the  pupil 


How  is  the  Zonula  Affected  ?  139 

contracts  to  such  a  degree  as  to  bring  the  anterior  capsule  in  contact  with  the 
iris,  or  apparently  near  to  that  point,  this  shaking  or  trembling  phenomenon  is 
no  longer  visible. 

In  an  interesting  case  of  aniridia  with  central  opacity  in  the  lens,  which 
was  recently  reported  by  Grossman  (B  325),  this  trembling  of  the  lens  at  one 
stage  was  unusually  well  shown. 

It  is  possible  with  proper  care  and  appliances  to  observe  a  slight  motion  of 
the  lens  even  in  a  normal  eye.  Thus  persons  accustomed  to  use  the  oph- 
thalmoscope or  trained  to  experiments  in  optics  can  increase  or  decrease 
their  accommodation  at  will.  Now  if  the  observer  focuses  the  anterior  surface 
of  the  lens  in  such  an  eye  with  a  Zeiss  loupe,  and  notices  carefully  the 
chagrin  already  described,  and  the  subject  be  asked  to  adjust  his  accommoda- 
tion first  for  the  distance,  then  for  the  near  point,  altering  it  rapidly,  and  at  the 
same  time  to  move  the  eye  quickly,  a  slight  tremulous  motion  can  often  be 
detected. 

The  foregoing  facts  undoubtedly  indicate  that  contraction  of  the  ciliary 
muscle  relaxes  the  zonula. 


FiG.  104. — Changes  in  the  position  of  the  lens  in 
Grossman' s  case  of  aniridia. 

A.  Position   of    the    lens   with   accommodation 
relaxed. 

B.  Position  of  the  lens  after  instillation  of  eserin. 
The  spot  in  the  center  is  a  small  point  of  capsular 

opacity. 

On  the  other  hand  certain  observations  indicate  that  the  zonula  is  made 
more  tense. 

(A)  The  entire  lens  sometimes  moves  upward  and  slightly  inward.     This 
was  well  shown  in  Grossman's  case  (Fig.  104). 

(B)  Some  think  that  tension  of  the  zonula  increases  the  convexity  of  the 
anterior  surface  (Fig,  105).     This  observation  by  Crzellitzer  (B  306-307)  and 
others  by  Stadfeldt  (B  309)  apparently    confirm  the  view  of  Tscherning  that 
tension  on  the  zonula  may  produce  an  anterior  lenticonus. 

The  method  of  Crzellitzer  (B  306)  of  holding  the  lens  is  suggestive  to  future 
students  of  the  question.  He  prepared  a  small  instrument,  as  represented 
in  Fig.  106,  whose  object  was  to  hold  a  lens  in  the  central  opening  in  such  a 
way  that  traction  could  be  made  at  its  edges  in  different  directions  at  the  same 
time.  The  lens  was  then  removed  from  the  eye  of  an  ox,  and  being  sus- 
pended in  the  center  in  the  inner  circle  of  the  instrument,  the  screw  B  was 
turned.  Under  these  circumstances  he  thought  that  while  the  posterior 


140 


How  is  the  Zonula  Affected? 


surface   remained  practically    stationary,  the  anterior    surface   became   more 
convex.     The  validity  of  this  experiment,  however,  is  denied  by  Hess. 

The  act  of  accommodation  in  the  eye  of  the  lower  animals  furnishes 
apparently  a  very  promising  field  from  which  to  collect  data  concerning  the 
behavior  of  the  ciliary  muscle  and  its  effect  upon  the  zonula.  This  is  especially 
true  of  reptiles,  batrachians,  etc.,  because  in  them,  muscle  action  persists  a 
considerable  time  after  the  eye  has  been  isolated  from  the  body.  This  has 

I 


FIG.  105. — Lens  of  the  ox,  twice  enlarged.  The  dotted 
line  indicates  the  form  which  the  crystalline  lens  assumes  : 
(A)  by  a  lateral  pressure  (B)  by  traction  exerted  on  the 
zonula.  The  arrows  indicate  the  direction  of  the  forces 
(Tscherning). 


FIG.  1 06. — Arrangement   for  making  tension   on   the   zonula  in  order  to 
observe  changes  in  the  lens  (Crzellitzer). 


How  is  the  Zonula  Affected? 


141 


been  carefully  studied  by  Beer  (B  319).  A  striking  demonstration  of  accom- 
modation in  the  eye  of  a  snake  was  made  by  him  at  a  meeting  of  the 
Ophthalmological  Section  of  the  New  York  Academy  of  Medicine  in 
October,  1904.  Briefly  stated,  the  method  consisted  in  decapitating  the  animal, 
quickly  removing  the  eye,  placing  it  beneath  a  microscope  with  its  lens  vertical 
and  about  in  the  center  of  the  field.  Then,  a  fine  wire  having  been  brought 
in  contact  with  the  muscles  which  adhered  to  the  side  of  the  globe,  a  current 
of  electricity  was  passed  through  the  eye  from  a  battery  of  three  small  cells. 
The  experiment  was  most  suggestive  and  interesting.  With  the  eye  in  this 
position  the  iris  and  lens  could  be  seen  distinctly.  Whenever  the  current  was 
closed,  the  pupil  immediately  contracted  and  the  lens  appeared  to  jump  for- 
ward as  its  anterior  surface  became  more  convex. 


FIG.  107  — Method  in  which  the  lens  is  suspended  by  the  above 
instrument. 

In  view  of  the  testimony  now  before  us,  it  is  almost  pre- 
sumptuous for  one  who  has  not  devoted  himself  for  a  con- 
siderable time  to  studying  these  phenomena  to  express  an 
opinion.  The  evidence  on  both  sides  is  here  given  with  the 
hope  that  it  may  induce  others  to  pursue  the  study  farther. 
From  the  facts  thus  far  collected  it  must  be  conceded,  how- 

A  B 


FIG.  108. — Relative  position  of  the  reflections  from  the  cornea  and 
from  the  two  surfaces  of  the  lens  during  the  act  of  accommodation. 
A  represents  their  situation  in  the  eye  accommodated  for  distance ; 
B,  in  the  eye  accommodated  for  near  objects.  In  both,  a  is  the 
image  reflected  from  the  cornea ;  b,  that  from  the  anterior  surface, 
and  c,  that  from  the  posterior  surface  of  the  lens  (Bonders). 

ever,  that  the  balance  of  evidence  favors  the  conclusion  that 
the  zonula  does  relax  and,  in  a  word,  that  the  so-called 
Helmholtz  theory,  perhaps  with  some  modification,  is  nearest 
to  the  truth. 


142 


Change  in  the  Anterior  Surface 


Fifth.  Whatever  the  condition  of  the  zonula  may  be,  it  is 
certain  that  the  anterior  surface  of  the  lens  becomes  more 
convex.  All  agree  on  this  point,  though  there  is  a  marked 
disagreement  as  to  the  exact  form  which  that  surface 
assumes.  To  determine  this,  we  depend  principally  upon 
the  reflections  from  the  anterior  surface  of  the  lens  during 
the  act  of  accommodation  (Fig.  108). 


Repose 


Accommodation 


FIG.  109. — Change  in  the  form  of  the  anterior  surface  of 
the  lens  shown  by  the  change  in  the  relative  position  of  the 
reflections  from  six  points  of  light.  Three  of  these  points 
are  in  one  horizontal  line,  and  three  other  points  of  light 
.directly  b.elow  are  also  in  a  horizontal  line.  The  diagram 
on  the  left  shows  the  position  of  these  six  reflections  when 
the  lens  is  in  a  state  of  repose.  The  six  points  in  the  dia- 
gram on  the  right  show  the  relative  position  of  these  reflec- 
tions when  the  lens  has  been  accommodated  to  a  near  point 
(Tscherning). 

Inasmuch  as  a  strongly  convex  mirror  gives  a  smaller 
image  than  one  of  less  curvature,  and  as  in  B  the 
image  reflected  from  the  anterior  surface  of  the  lens  has 
become  smaller  than  it  was  in  A,  the  inference  is  that  the 
curvature  of  the  anterior  surface  increased.  Helmholtz 
supposed  that  this  increase  in  the  curvature  was  equal  in 
all  parts  of  the  anterior  surface. 

A  more  exact  study  of  these  reflections  from  the  anterior 
surface  of  the  lens,  however,  tends  to  show  that  the  increase 
in  the  convexity  is  really  unequal,  being  greater  in  the 
center  than  near  the  equator.  Thus  Fig.  109  gives  the 
reflections,  from  the  anterior  surface  only,  of  the  lens,  of  six 
different  lights:  a  is  when  the  eye  is  adjusted  for  dis- 
tance, and  b  in  accommodation. 


Posterior  Surface  of  the  Lens 


'43 


FIG.  no. — Change  in  the 
form  of  the  lens  during  ac- 
commodation. The  con- 
tinued line  indicates  the 
shape  of  the  lens  in  a  state 
of  repose.  The  dotted  line 
shows  the  shape  of  the  lens 
with  seven  diopters  of  ac- 
commodation (Tscherning). 


These  reflections  from  the  upper 
horizontal  line  of  lights  do  not  ap- 
proach in  an  equal  degree  those  from 
the  lower  horizontal  line,  but  instead, 
each  line  of  the  reflections  tends  to 
arrange  itself  in  a  curve  with  its 
convexity  toward  the  center  of  the 
lens.  For  the  proper  elaboration  of 
this  point  the  student  must  turn  to 
the  article  by  Tscherning  (B  291). 
The  evidence  tends  to  show  that  a 
real  anterior  lenticonus  is  produced, 
as  Tscherning  states  (Fig  1 10). 

Sixth.  The  posterior  surface  pos- 
sibly increases  its  convexity.  Helm- 
holtz  considers  that  probable. 
Tscherning  is  doubtful,  and  the  expe- 
riments of  Hensen  and  Voelckers 
(B  289),  which  are  cited  by  Landolt 
(B  328)  as  to  the  movement  backward 
of  the  posterior  capsule,  are  not  as 
conclusive  as  they  appear.  In  a  word, 
it  is  not  certain  that  the  posterior 
surface  moves  at  all. 

It  is  worth  while  to  consider  the 
act  of  accommodation  thus  in  some 
detail  in  order  to  appreciate  what  we 
do  know,  and  what  we  do  not  know 
concerning  it.  Evidently  the  ques- 
tion whether  the  zonula  is  contracted  or  relaxed  is  not  of  as 
great  practical  importance  as  might  at  first  appear.  As  all 
agree  that  these  changes  depend  primarily  upon  a  contraction 
of  the  ciliary  muscle,  we  see  how  accommodation  is  essen- 
tially an  active,  and  not  a  passive  process.  The  amount 
which  can  be  exerted  may  depend  on  the  age  of  the  person, 
on  the  condition  of  his  refraction,  and  on  various  other 
factors,  but  it  always  demands  an  effort.  We  shall  see  later 
that  abnormal  contractions  and  relaxations  of  the  ciliary 
muscle  produce  forms  of  muscle  imbalance  which  are  of  much 


144  Range  of  Accommodation 

importance  clinically,  and  we  can  deal  with  these  more  in- 
telligently after  even  this  hasty  review  of  our  data  concern- 
ing the  physiological  act  of  accommodation. 

§  2.  The  Range  of  Accommodation  is  the  well- 
known  term  to  denote  the  amount  of  accommodation  of 
which  an  eye  is  capable.  Thus  if  an  emmetrope  whose 
accommodation  is  at  rest  when  looking  at  a  distant  object 
can  also  see  a  point  distinctly  which  is  only  one-tenth  of  a 
meter  in  front  of  the  eye,  we  say  that  he  has  a  range  of 
accommodation  of  ten  diopters.  This  is  often  represented 
diagrammatically,  as  in  Fig.  m,and  a  similar  line  will  be 
used  frequently  in  the  graphic  representation  of  muscle 
balance  and  imbalance. 


1    {V     '     '    J 

...             •-»,- 

R 

FIG.  ill. — Diagram  showing  the  range  of  accommodation,  p  is  the 
near  (proximate)  point.  P,  distance  of  the  proximate  point  from  the  nodal 
point  of  the  eye.  r  is  the  remote  point.  R  is  the  distance  of  the  remote 
point  from  the  nodal  point  of  the  eye. 

If,  however,  the  person  has  a  hypermetropia  of  two  diop- 
ters, he  must  exert  that  amount  of  accommodation  in 
order  to  see  even  a  distant  object  distinctly,  and  if  he  can 
also  see  a  test  object  at  a  distance  of  one-tenth  of  a  meter, 
then  his  range  of  accommodation  is  twelve  diopters.  These 
well-known  facts  are  referred  to  only  for  the  sake  of  com- 
pleteness. 

Of  the  various  methods  for  measuring  the  nearest  point 
of  clear  vision,  the  most  usual  one  now,  is  to  ascertain  the 
nearest  point  at  which  suitable  test  letters  can  be  seen  dis- 
tinctly. Instead  of  letters  Bonders  (B  260)  used  a  series  of 
fine  wires,  to  which  a  tape  measure  was  attached,  and 
Landolt  suggested  placing  around  a  candle  flame  a  cylinder 
perforated  with  small  openings.  The  nearest  point  at 
which  the  individual  could  distinguish  these  openings 
as  separate  dots  indicated  the  proximate  limit  of  the 
range  of  accommodation  (Fig.  112).  When  test  letters  are 


How  a  Lens  Affects  Accommodation 


145 


exactly  constructed,  however,  and  reduced  to  the  proper 
size  by  photography,  if  necessary,  they  are  as  accurate  as  any 
other  test  objects  for  the  near  point,  and  much  the  most 
convenient  for  clinical  purposes. 

The  range  of  accommodation  is 
modified  by  age.  Although  this 
fact  also  is  well  known,  attention  is 
called  to  it  here  because  it  will  be 
necessary  to  refer  frequently  to  its 
bearing  on  certain  abnormal  condi- 
tions of  accommodation  occurring 
with  presbyopia  which  give  rise  to 
muscle  imbalance.  In  this  connec- 
tion it  is  worth  while  to  recall  the 
familiar  diagram  of  Bonders.  In 
Fig.  1 1 3  the  figures  at  the  top  repre- 
sent the  age  of  the  individual,  those 
on  the  left  represent  the  range  of 
accommodation  in  diopters  from 
infinity  to  20  diopters  above,  or 
from  infinity  to  8  diopters  below. 
From  this  we  see  that  at  10  years 

of  age  the  normal  eye  can  accommodate  about  14  diopters, 
at  30  years  of  age  about  7  diopters,  and  at  55  less  than  2 
diopters.  At  75  the  power  of  accommodation  is  practically 
lost,  and  after  that  a  weak  convex  glass  may  be  necessary 
for  clear  vision  even  in  the  distance.  The  foregoing  relates 
to  accommodation  with  one  eye  only,  or  what  Donders  called 
monocular  accommodation.  He  made  a  distinction  be- 
tween this  and  binocular  accommodation,  but  the  basis  of 
this  difference  has  recently  been  disputed  by  Hess  (B  329). 
A  discussion  of  that  point  would  require  too  long  a  di- 
gression here.  Suffice  it  to  say  that  the  binocular  range  can 
probably  be  considered  the  same  as  the  monocular. 

§  3.  How  a  Lens  before  the  Eye  Affects  its  Focal 
Power  and  therefore  its  Accommodation. — In  testing 
for  relative  accommodation,  as  will  be  done  later,  if  we  wish 
to  be  exact  we  must  be  ready  to  calculate  the  effect  which 


FIG.  112. — A  simple  ar- 
rangement for  determining 
the  near  point  (Landolt). 


146 


How  a  Lens  Affects  Accommodation 


a  glass  of  a  given  strength  has  on  the  focal  distances  of  the 
eye.  But  as  this  affects  the  power  of  accommodation  it 
will  cause  less  confusion  to  dispose  of  the  question  at  this 


10    15    20   25   30  35  40   45   50   55  60  65  70  75  80 


20 
18 

16 
14 
13 
11 
10 
8 
6 
5 
3.26 
W 

00 

16 
3.25 
5 

e 

B 

X 

• 

\ 

S 

\ 

\ 

\ 

\ 

\ 

\ 

x 

^v 

v>>- 

hi 

—  « 

= 

—  — 

"^ 
••  — 

**-^ 

••  — 

/» 

;=». 

"J1"1 

i^c 

•*-. 

r> 

1 

FIG.  113. — Diagram  showing  the  range  of  accom- 
modation at  various  ages. 


point.  In  this  connection  most  readers  will  recall  the  for- 
mula given  by  Bonders,  (B  260,  p.  144).  But  as  there  are 
several  misprints  in  that  part. of  the  English  edition,  the 
method  of  calculation  is  given  here. 

In  Figures  114,  115,  116  let  P  represent  the  distance  from  the  nodal  point 
K  to  the  point/,  and  P'  the  distance  from  K  to  the  point/'  and  d  the  distance 
of  the  lens  from  the  nodal  point.  In  these  formulas  p  is  the  point  looked  at, 
while/'  is  the  point  for  which  the  eye  is  adjusted.  -—  will  be  the  number  of 

the  glass  1,  and  F  its  focal  distance.  We  must  remember  that  the 
power  of  a  lens  or  system  is  equal  to  the  difference  of  the  reciprocals  of 


How  a  Lens  Affects  Accommodation 


the  distance  of  any  two  conjugate  points  when  those  two  points  are  on 
the  same  side  of  the  lens,  and  therefore  have,  as  we  say  in  optics,  "  like 
signs  "  and  equal  to  the  sum  of  the  reciprocals  when  the  two  points  are  on 


R      "4 


R  "5 


116 


FIGS.  114,  115,  116.  —  How  a  lens  before  the  eye  affects  its  focal  distance. 


opposite  sides  of   the  lens  —  that  is,  have  opposite  signs.      If  a  convex  lens  be 
placed  before  the  eye,  then  the  formula  follows  : 


d  ~  P'  —  d 


(i) 


_ 

P'_d 


—  d 


_ 

—  d~P'  — 


Or  it  may  be  said  that  if  the  rays  coming  from  /  (Fig.  115)  were  given  addi- 
tional convergence  by  the  glass  lens  -==;  then  they  would  appear  to  come  from 

the  farther  point/'. 

Let  us  next  see  what  happens  when  a  concave  glass  is  placed  before  the  eye 
(Fig.  116).  When  the  two  points//'  are  again  on  the  same  side  of  the  lens 
they  are  -f-  signs,  and  therefore  the  power  of  the  system  of  the  glass  lens  and 


148          Measurement  of  Pupillary  Reaction 

the  lens  of  the  eye  is  expressed  by  the  same  formula  (i).     But  in  this  case 

—  -  -  being  larger  than  -  -  -  the  value  of  the  sign  of  -=-  is  negative,  as 
P  —  d  P  —  d  r 

follows  : 

I  I  I  I  I  i 

or 


P'  —  d  ~~  P~^d       ~F~      P^d  ~  P'  —  d         F" 
That  is  to  say,  if  the  rays  emanating  from  /  were  made  more  divergent  by  the 

glass  lens  \  —  =-  J  ,  then  the  rays  would  come  to  a  focus  nearer  by  —  namely,  at/'.' 
This,  with  the  preceding  formula,  we  will  have  occasion  to  use  in  the  meas- 
urement of  relative  accommodation. 

§  4.  Measurement  of  the  Pupillary  Reaction  for 
Physiological  and  Clinical  Purposes.  —  Whoever  under- 
takes to  examine  the  literature  of  pupillometry  is  soon 
impressed  with  three  facts  : 

First,  by  the  large  number  of  ingenious  and  careful 
studies  which  have  been  made  to  determine  the  size  of  the 
pupil  ; 

Second,  by  the  comparatively  small  number  to  ascertain 
the  rapidity  and  degree  of  variations  in  its  size,  or  the  causes 
which  produce  these  variations; 

Third,  by  the  fact  that,  while  these  methods  of  investiga- 
tion are  well  suited  to  laboratory  experiment,  they  are  so 
poorly  adapted  to  clinical  use  that  practitioners  do  not  avail 
themselves  of  the  results  obtained,  in  spite  of  the  import- 
ance of  the  symptomatology  of  the  pupil. 

In  this  connection  it  is  only  possible  to  indicate  briefly 
the  various  factors  in  the  problem  of  pupillometry,  and  to 
refer  to  an  instrument  which  has  proved  of  at  least  some 
assistance  in  studying  this  question. 

In  all  measurements  of  the  pupil  there  are  of  course  two 
aspects  of  the  problem,  the  pupil  and  the  instrument  with 
which  it  is  measured.  The  variations  of  the  former  and  the 
imperfections  of  the  latter  constitute  the  difficulties  pre- 
sented. It  should  be  remembered  at  the  outset  that  the  size 
of  the  pupil  taken  by  itself  is  of  comparatively  slight  im- 
portance, varying  as  it  does  in  different  individuals  and 
being  smaller  in  advanced  life  than  in  youth.  Thus  in  young 
people  from  fifteen  to  twenty  it  is  about  four  (4.1)  millime- 
ters, and  in  persons  of  fifty  or  more,  about  three  millimeters 
in  diameter. 


Measurement  of  Pupillary  Reaction  149 

With  this  fact  determined  as  to  the  size  of  the  pupil,  let 
us  review  briefly  the  main  causes  which  produce  changes  in 
its  diameter. 

First,  we  naturally  think  of  variations  due  to  intensity  of 
illumination.  That  means  of  course  that  any  measurements 
to  be  accurate  must  have  a  definite  relation  to  photometric 
standards.  For  very  exact  measurements  a  photometer  is 
an  undoubted  necessity,  but  Schirmer  says  (B  349,  p.  12) 
that  "if  the  window  be  covered  with  two  white  curtains, 
which  either  alone  or  together  can  be  drawn  down,  the 
amount  of  illumination  can  be  sufficiently  regulated."  This 
is  also  sufficient  for  most  of  the  clinical  examinations  made 
to  determine  whether  or  not  the  iris  still  retains  a  consider, 
ble  amount  of  mobility. 

Second,  we  know  that  the  pupil  also  contracts  with  all 
efforts  at  accommodation,  and  this  means  that  in  any  of 
these  measurements  the  person  must  look  at  a  distant 
object. 

A  third  group  of  causes  tending  to  vary  the  size  of  the 
pupil  relates  to  the  respiration  and  circulation.  We  know 
that  the  pupil  dilates  with  deep  inspiration  and  that  to  a 
certain  extent  it  is  influenced  by  variations  in  the  pulse  and 
blood  pressure.  With  a  little  care,  however,  an  intelligent 
subject  can  be  taught  to  breathe  so  regularly  that,  barring 
pathological  conditions  of  the  circulation,  this  group  of 
causes  can  be  eliminated  as  a  factor  in  the  variation  of  the 
size  of  the  pupil. 

Finally,  varying  conditions  of  the  nervous  system,  espe- 
cially those  involving  the  sympathetic,  produce  differences 
in  the  size  of  the  pupil.  These  may  be  the  temporary 
effect  of  fear,  surprise,  or  other  emotions,  or  the  more  lasting 
changes  from  lesions  of  the  motor  oculi.  Moreover,  each 
of  these  different  groups  of  causes  is  often  influenced  by 
other  causes  which  are  still  unknown. 

Having  thus  glanced  hastily  at  the  principal  factors 
which  tend  to  change  the  size  of  the  pupil,  let  us  pass  next 
to  an  instrument  with  .which  measurements  of  these  varia- 
tions can  be  made — exactly  enough,  at  least,  to  assist  in 
conclusions  physiological  as  well  as  clinical. 


150 


The  Ophthalmic  Microscope 


The  accompanying  illustration  (Fig.  117)  shows  a  micro- 
scope arranged  especially  for  this  purpose  (B  345). 

The  tube  is  mounted  horizontally  on  a  firm  tripod  at- 
tached  to  an  upright  bar.  As  this  bar  can  be  lengthened 
or  shortened,  the  microscope  can  be  lowered  or  raised,  and 


FIG.  117. — Horizontal  ophthalmic  microscope. 

swings  horizontally  on  the  axis  of  the  bar.  There  is  a 
hinge  joint  allowing  the  elevation  or  depression  of  the  tube 
at  any  angle,  and  by  means  of  a  rack-and-pinion  adjust- 
ment it  can  be  pushed  forward  or  backward.  In  a  word, 
the  instrument  can  be  brought  at  once  into  any  position 
desired. 

There  are  two  eye -pieces.  One  has  a  focal  length  of 
about  nine  centimeters,  and  gives  a  minimum  amplifica- 
tion of  twenty  diameters  to  a  maximum  of  fifty,  according 
to  the  position  of  the  draw  tube.  The  other  eye-piece  has 
a  focus  of  2.5  centimeters,  giving  a  minimum  amplification 
of  fifty  diameters  or  maximum  of  about  one  hundred  and 
twenty-five.  A  micrometer  eye-piece  enables  changes  in 
the  size  of  the  pupil  to  be  measured  with  accuracy. 

When  making  an  examination,  the  patient  faces  the 
instrument,  the  eye  being  approximately  near  the  focus  of 
the  objective.  The  results  are  best  if  the  chin  is  placed  on  a 


The  Ophthalmic  Microscope  151 

head-rest,  with  the  teeth  fixed  in  a  Helmholtz  bit.  But  for 
the  usual  tests  for  physiological  or  for  clinical  purposes  it  is 
sufficient  to  have  the  person  rest  his  elbows  on  the  table, 
and  then  support  his  chin  on  his  hands. 

Ordinarily  daylight  is  sufficient,  but  if  specially  good 
views  are  desired  they  can  be  obtained  by  bringing  a 
shaded  electric  light  within  a  foot,  or  two  of  the  patient's 
head,  or  by  allowing  the  light  coming  through  a  double 
convex  lens  to  fall  obliquely  on  the  eye.  While  these 
details  are  given  concerning  the  most  desirable  position  of 
the  patient  and  the  degree  of  illumination,  it  should  be 
borne  in  mind  that  such  care  is  by  no  means  essential  for 
clinical  work. 

The  patient  having  been  seated,  the  head  adjusted,  and 
the  light  arranged,  the  objective  is  brought  within  a  few 
inches  of  the  eye,  and  almost  immediately  the  direction  and 
the  focus  are  obtained.  The  view  presented  is  quite  striking 
to  one  who  for  the  first  time  uses  a  microscope  in  this  way, 
even  though  with  the  ordinary  pocket  lens  we  are  accus- 
tomed to  see  something  similar  to  it  every  day. 

When  the  pupil  is  somewhat  dilated  in  a  diffused  mod- 
erate light  we  usually  find  it  freely  movable,  so  that  within 
thirty  seconds  it  is  possible  to  count  at  least  two  or  three 
strong,  or  what  may  be  called  maximum  contractions,  four 
or  five  moderate  or  minor  contractions,  and  as  many  or 
more  very  slight  or  minimum  contractions.  Usually  each 
contraction  is  followed  by  a  corresponding  dilatation,  but 
this  is  not  always  the  case,  two  or  three  contractions  some- 
times following  each  other  in  succession  before  the  occurrence 
of  a  dilatation  equal  to  or  greater  than  the  three  together. 
Naturally  there  are  decided  variations  both  in  the  number 
and  the  degree  of  these  changes,  but  a  little  experience 
enables  one  to  recognize  them,  and  to  establish  for  himself, 
at  least,  a  normal  standard  which  is  interesting  and  new  to 
one  familiar  only  with  those  movements  of  the  iris  which 
are  visible  to  the  naked  eye. 

Of  course,  the  contractions  of  the  iris  are  by  no  means 
always  the  same,  its  behavior  in  one  thirty  seconds  being 
entirely  different  from  that  occurring  in  the  next  half  minute. 
Usually,  however,  after  a  few  careful  observations  it  is 


152  Pupillary  Reaction 

possible  to  determine  the  characteristics  of  any  given  pupil, 
whether  normal  or  abnormal,  and  at  least  approximately 
the  character  and  degree  of  any  variation  from  the  average. 
In  this  way  it  is  possible  to  differentiate  the  following  types 
of  pupiljary  reaction. 

First.  What  may  be  considered  the  normal  pupil,  this 
being  about  such  a  diameter  as  just  described,  and  one 
where  the  reactions  also  are  of  the  degree  and  frequency 
already  indicated  as  normal. 

Second.  A  pupil  unnaturally  large  or  unnaturally  small 
with : 

A.  An  unusually  large  number  of  maximum  contractions. 

B.  An  unusually  small  number  of  maximum  contractions. 

C.  An  unusually  large  number  of  minimum  contractions. 

D.  An  unusually  small   number  of   minimum   contrac- 
tions, or 

E.  With  combinations  of  these  types. 

The  thanks  of  the  profession  are  due  to  such  painstaking 
workers  in  this  field  as  Schirmer  (B  349),  Bielschowski 
(B  350),  and  others,  but  most  of  these  interesting  studies 
have  been  made  as  laboratory  experiments  and  rather 
to  determine  the  physiological  size  of  the  pupil  than  its 
average  behavior  under  abnormal  conditions.  When,  how- 
ever, greater  care  in  measuring  the  pupil  is  taken  by  the 
ophthalmologist,  he  will  be  surprised  to  see  how  much  has 
escaped  his  attention,  and  he  will  be  apt  to  strive  for  greater 
exactness  in  observing  pupillary  reaction. 

Later  in  our  study  of  the  anomalies  of  accommodation 
special  effort  will  be  made  to  separate,  whenever  it  is  pos- 
sible, cases  of  excessive  accommodation  from  those  in 
which  the  power  of  accommodation  is  insufficient.  In 
doing  this  we  shall  find,  as  a  rule,  that  in  excessive  accommo- 
dation the  pupil  is  comparatively  small  and  the  variations  in 
its  size  neither  many  nor  great.  On  the  other  hand,  when 
the  power  of  accommodation  tends  to  be  actually  insufficient, 
or  insufficient  in  relation  to  the  resistance  to  be  overcome 
in  that  individual  case,  the  pupil  is  more  apt  to  be  rather 
large  and  to  show  considerable  variations.  The  type  of 
the  latter  is  seen  sometimes  in  the  rather  large  pupil  of 
hypermetropia,  that  condition  of  the  refraction  producing 


ASTIGMA  TIC  ACCOMMODA  TION 


'53 


what  may  be  called  a  relative  insufficient  accommodation* 
The  clinical  importance  of  this  will  be  stated  later  and 
should  not  be  overestimated  because  of  its  mention  now. 

§  5.  Irregular  (Astigmatic)  Contraction  of  the  Ciliary 
Muscle. — Most  of  the  older  text-books  teach  that  when  a 
motor  impulse  is  sent  to  the  ciliary  region  all  of  the  mus- 
cular fibers  contract  in  absolutely  the  same  degree  at  the 
same  time — in  other  words,  that  the  force  exerted  upon  the 
edge  of  the  capsule  is  exactly  the  same  in  all  directions. 
Some  of  the  most  recent  observers  still  incline  to  doubt 
the  existence  of  astigmatic  accommodation.  But  there  are 
several  facts  which  indicate  very  strongly  that  such  con- 
traction of  the  ciliary  process  does  occur,  at  least  in  certain 
cases. 

i st.  It  is  entirely  possible  from  the  anatomical  arrange- 
ment. 

2d.  Exact  measurements  of  the  contraction  in  other 
muscles  show  that  there  is  frequently  a  difference  in  the 
degree  of  tension  in  different  fibers. 

3d.  As  certain  branches  of  the  third  nerve  are  some- 
times paretic  or  "  insufficient,"  leaving  the  muscles  supplied 
by  other  branches  in  an  entirely  normal  condition,  so  it  is 
probable  that  part  of  the  filaments  which  go  to  the  ciliary 
muscle  may  also  remain  in  a  normal  condition,  while  others 
may  be  less  or  more  active,  thus  producing  irregular  action 
on  the  zonula. 

4th.  A  significant  fact  pointing  to  the  ability  of  the 
ciliary  muscle  to  overcome  an  existing  astigmatism  is  met 
with  frequently  in  the  consulting  room.  For  example,  we 
find  that  although  the  individual  can  read  $  without  diffi- 
culty and  insists  that  the  lines  of  the  astigmatic  chart  are 
all  of  equal  size  and  clearness,  yet  examination  with  the 
ophthalmometer,  with  the  ophthalmophacometer,  and  with 
the  shadow  test  shows  the  presence  of  a  very  decided  astig- 
matism. Astigmatic  accommodation  is  the  simplest  and 
most  rational  method  of  explaining  this  difference.  Or  we 
may  find  subjectively  a  low  degree  of  astigmatism  in  cer- 
tain meridians,  whereas  the  objective  tests  indicate  that  its 
axes  are  in  still  other  meridians. 


1 54  Cycloplegics  and  Mydriatics 

Moreover,  it  can  be  demonstrated  experimentally  that  the 
increased  convexity  of  the  lens  during  accommodation  is  of 
such  a  form,  sometimes  at  least,  as  to  compensate  for  an 
irregular  curvature  of  the  cornea.  This  point  was  first 
established  by  Dobrowolsky  (B  352).  He  measured  the 
amount  of  corneal  astigmatism  in  his  own  eye,  and  then 
ascertained  by  other  measurements  that  the  form  which  the 
lens  assumed  during  accommodation  was  such  as  to  neutralize 
this,  at  least  to  a  certain  extent.  Similar  observations  have 
been  made  by  Woinow  (B  353). 

5th.  Another  observation  which  points  towards  the 
existence  of  astigmatic  accommodation  is  also  of  a  clinical 
character,  but  does  not  depend  upon  the  clearness  of  the 
image.  This  is  the  sensation  of  the  patient.  It  is  an  every- 
day experience  that  the  correction  of  an  astigmatism  less- 
ens discomfort  and  ocular  headache.  The  relief  indicates 
that  the  symptoms  were  due  to  an  effort  of  some  of  the 
fibers  of  the  ciliary  muscle  to  do  what  the  glass  did  for 
them. 

In  view  of  the  testimony  thus  presented,  it  seems  that 
the  balance  of  evidence  is  strongly  in  favor  of  the  conclu- 
sion that  there  does  exist  an  astigmatic  accommodation 
which  in  some  individuals,  at  least,  is  not  only  measurable  but 
exceedingly  well  marked.  This  is  not  simply  in  what  we 
may  call  physiological  conditions,  but  in  the  pathology  of 
muscle  imbalance  irregular  accommodation  is  undoubtedly 
an  important  factor. 

§  6.  How  to  Measure  the  Effects  of  Cycloplegics  and 
Mydriatics. — When  considering  the  anomalies  of  accom- 
modation we  shall  find  frequent  references  to  the  effects  of 
atropin  or  eserin,  especially  after  the  employment  of 
so-called  minimum  doses.  It  is  proper,  therefore,  to  enter 
into  a  little  detail  concerning  the  form  in  which  these 
drugs  can  best  be  used,  and  their  physiological  effects. 

At  the  outset  it  should  be  observed  that  solutions  are 
uncertain  and  not  to  be  relied  upon  for  exact  measurement. 
No  matter  how  accurately  a  solution  is  made,  it  is  apt  to 
undergo  changes  in  a  short  time  by  evaporation  or  deteriora- 
tion. Even  when  it  is  perfectly  fresh  we  cannot  be  certain 


Cycloplegics  and  Myotics 


155 


that  a  definite  amount  reaches  the  conjunctiva  or  is  absorbed. 

Again,  drops  differ  in  size,  and  even  when  a  single  drop 
of  average  size  is  applied  to  the  conjunctiva,  the  tears  may 
cause  more  or  less  to  be  washed  away,  or  it  may  pass  off  at 
once  through  the  canaliculi  if  they  happen  to  be  unusually 
large.  A  much  more  exact  method  of  application  is  by 
means  of  the  ophthalmic  discs  now  made  by  several  reliable 
pharmacists.  In  the  experiments  here  referred  to  these 
discs  have  been  used. 

When  attempting  to  observe  the  effect  of  a  certain  dose  of 
a  cycloplegic  or  myotic  it  is  necessary  to  have  proper  appli- 
ances for  taking  the  readings  and  recording  the  results. 
Thee?  are: 


FlG.  118. — Testing  the  nearest  point  of  clear  vision. 

(a)  Types  for  testing  the  nearest  point  of  clear  vision.  A 
series  of  such  letters  has  been  already  described,  the  smallest 
of  which  should  be  seen  at  a  distance  of  33  centimeters. 
When  we  wish  to  test  young  subjects,  or  those  who  have  an 
accommodation  greater  than  three  diopters,  we  should  theo- 


156  Cycloplegics  and  Myotics 

retically  make  use  of  still  smaller  letters.  As  such  types  are 
used  for  measuring  the  range  of  relative  accommodation, 
they  will  be  found  described  in  the  chapter  relating  to  that 
subject.  The  fact  is,  however,  that  the  smallest  letters  of 
the  series  already  figured,  are  sufficient  for  most  of  the 
measurements  necessary  in  the  consulting  room. 

(b)  We  require  also  a  suitable  measure  of  the  distance 
at  which  the  type  is  held.     The  arrangement  used  for  this 
purpose  consists  essentially  of  the  rack  of  a  stereoscope  which, 
on  each  side  of  the  horizontal  bar,  has  the  uprights  to  hold 
the     card,    and     below    the  center  of  the  cross-bar,  a  firm 
handle.     The    latter   is   bifurcated    near    its  attachment  to 
the    cross-bar,   and    between     the    arms    there    is    placed 
a  measure  graduated   in  centimeters  and  in  diopters  and 
rolled    on    a   spring.     The   ring  fixed   on  the  end  of  the 
measure  has  a  second  handle  attached  to  it,  that  one  being 
much  smaller  than  the  first  (Fig.  118). 

(c)  A  measure  of  the  size  of  the  pupil.     The  most  accu- 
rate one  for  this  is  the  horizontal  ophthalmic  microscope 
already  described,  but  for   all  ordinary  purposes  it  is  quite 
sufficient  simply  to  hold  a  millimeter  measure  in  front  of  the 
eye  and  as  close  to  it  as  possible. 

(d)  Finally,  the  record  is   kept    best   on  charts  divided 
into   squares.      A   convenient    form    for   these   is   seen    in 
Figure  119.     This  indicates  the  number  of   minutes  after 
the  application  is  made,  while  the  other  blank  (Fig.  120) 
shows  the  number  of  hours  (or  of  days)  required  for  the 
disappearance  of  the  effects  of  the  drug.     While  these  charts 
are  extended  upward  to  allow  for  twenty  diopters  of  accom- 
modation, such   measurements   evidently   cannot  be  made 
with  ordinary  type,  although  with  full  doses  of   eserin  in 
young  people  the    near  point    approaches  nearer   than   is 
ordinarily  supposed.     For  clinical  purposes,  therefore,  espe- 
cially when  recording  the  effects  of  a  cycloplegic,  only  that 
portion  of  the  chart  is  necessary  which  is  near  the  zero  line 
and  they  are  thus  represented  in  most  of  the  diagrams.     It 
would  be  unnecessary  detail  to  illustrate  such  simple  blanks 
were  it  not  that  in  careful  clinical  work  these  are  as  useful  as 
the  blanks  for  recording  the  field  of  vision. 


Cycloplegics  and  Myotics  157 

Before  beginning  an  observation,  it  is  essential  to  determine 
the  refraction  of  the  eye  and  the  condition  of  the  accommo- 
dation. For  the  novice,  it  is  well  to  commence  a  measure- 
ment just  at  the  beginning  of  the  hour  or  at  some  multiple 
of  ten  minutes  past  the  hour,  for  then  less  confusion  arises 
concerning  the  number  of  minutes  after  the  application. 

The  details  of  such  a  test  are  as  follows:  Having  applied 
the  disc  to  the  conjunctival  sac,  it  is  unnecessary  to  make 
any  measurement  for  the  first  ten  or  twelve  minutes.  After 
that,  it  should  be  repeated  about  every  five  or  ten  minutes 
or  more  frequently  for  at  least  half  an  hour,  a  longer  interval 
being  sufficient  for  a  minimum  or  a  weak  dose  than  for  a 
full  one.  The  procedure  consists  simply  in  ascertaining 
the  nearest  point  at  which  the  patient  can  read  the 
finest  type.  The  details  of  the  measurement  itself  are 
simple.  Let  us  suppose  that  the  left  eye  is  to  be  tested. 
The  person  under  examination  closes  the  right  eye,  takes  in 
his  left  hand  the  small  handle  to  which  the  tape  of  the 
measure  is  attached,  the  end  of  the  measure  being  held 
near  the  outer  canthus  and  close  to  it.  At  the  same  time  he 
holds  the  larger  handle  in  his  right  hand  and  approaches  the 
card  to  the  nearest  point  at  which  the  smallest  letters  can 
be  distinguished.  This  distance  can  be  immediately  read 
off  on  the  tape  in  centimeters  or  in  diopters.  Thus  when  a 
young  person  with  normal  eyes  recognizes  print  No.  0.25 
at  a  distance  of  25  centimeters,  of  course  he  exerts  4  diop- 
ters of  accommodation.  If  he  can  read  correspondingly 
smaller  print  at  20  centimeters,  then  he  exerts  5  diop- 
ters, etc.  When  the  atropin  begins  to  have  its  effect, 
as  the  lens  becomes  less  convex  and  the  near  point  begins 
to  recede,  it  is  found  that  this  first  print  is  held  farther  and 
farther  away  until  it  becomes  impossible  for  the  person  to 
read  it  at  all.  Evidently,  therefore,  we  must  pass  to  the  next 
larger  size  of  the  test  type.  If  he  can  see  print  0.5  at  50 
centimeters  he  can  still  exert  2  diopters  of  accommoda- 
tion, and  so  on  until  finally,  when  he  only  sees  print  6.  at  6 
meters,  the  accommodation  is  entirely  relaxed.  This  is  all 
simple  enough  in  theory.  The  practical  fact  is,  however, 
that  in  making  these  tests  it  almost  never  occurs  that  the 


158 


Cyloplegics  and  Myotics 


ability  to  read  a  given  type  corresponds  with  the  reces- 
sion of  the  near  point.  This  is  for  the  reason  that  we  sel- 
dom find  an  absolutely  emmetropic  eye. 

In  making  these  measurements  we  often  find  that  the 
subject  can  read  the  0.25  print  at  25  centimeters  or  0.50  at 
50  centimeters.  But  as  the  relaxation  of  the  accommodation 


19 
18 
17 
16 
15 
14 
13 
12 
1  1 
10 
9 
8 
7 
6 
5 
4 
3 
2 
1 

"? 

2 
3 

* 
5 
6 

7 
8 

)   ; 

,  i 

0  1 

5  2 

02 

53 

03 

5  1 

Ot 

55 

0  5 

56 

06 

57 

0  7 

58 

0  6 

59 

09 

5  1 

0 

1 

0 

I 

0 

1 

iQ 

i 

0 

Name 
Day 


Age 


Drug  used 


FIG.  119.— Blank  on  which  to  record  the  immediate  effect  of  a  cycloplegic 
or  myotic. 

proceeds  he  cannot  read  the  I.  print  at  one  meter.  In  that 
case,  the  only  thing  to  do  is  to  make  note  at  the  top  of  the 
chart  of  the  smallest  print  which  can  be  read  at  that  dis- 
tance, both  with  and  without  correction.  Then,  when 
plotting  the  results,  account  must  be  taken  of  these  varia- 
tions from  the  normal  condition. 

In  spite  of  the  utmost  care,  there  are  certain  sources  of 
error  in  these  measurements.  We  are  not  certain  that  the 
manufacturer  of  the  medicated  discs  is  accurate  in  his  meth- 


Cycloplegics  and  Myotics 


159 


ods,  and,  even  if  that  be  the  case,  there  is  always  present 
a  degree  of  susceptibility  to  the  drug,  which,  as  we  know, 
varies  in  different  individuals.  When,  however,  all  these 
sources  of  error  are  taken  into  account,  we  find  them  in 
practice  so  slight  that  with  a  little  care  there  is  quite  a 
remarkable  degree  of  regularity  in  the  curves  obtained. 


FIG.  120.— Blank  on  which  to  record  the  effect  of  a  cycloplegic  or  myotic 
after  the  first  day  or  first  hour. 

§  7.  Physiological  Effect  of  Atropin  Sulphate  in  Full 
Doses.— In  that  most  interesting  volume  by  Darwin  on 
Insectiverous  Plants,  he  mentions  (p.  173)  the  information 
given  to  him  by  Donders  concerning  the  effect  upon  the  iris 
of  a  dog  of  a  millionth  of  a  grain  of  atropin.  The  fact  that 
a  naturalist  of  such  acumen  should  draw  his  facts  from  the 
field  of  ophthalmology  suggests,  in  a  way,  why  ophthal- 
mologists may  do  well  to  seek  for  other  facts  concerning 
this  same  class  of  drugs,  besides  those  with  which  most 
practitioners  are  familiar.  As  these  drugs  which  paralyze 


i6o 


Full  Dose  of  Atropin  Sulphate 


the  accommodation  are  used  so  frequently,  it  is  desirable  first 
to  recall  the  physiological  action  of  a  full  dose  of  one  of  them, 
— atropin  sulphate,  for  example — if  only  to  agree  as  to  what 
the  term  "  full  dose  "  means.  After  that,  we  can  study  with 
advantage  the  effect  of  smaller  or  "  minimum "  doses, 
because  later  we  shall  wish  to  use  these  to  ascertain  whether 
they  produce,  on  a  given  ciliary  muscle,  a  reaction  more  or 
less  prompt  and  complete  than  normal. 

In  the  classical  experiments  made  by  Kuyper  and  Don- 
ders  (B  379)  with  belladonna,  "they  always  spoke  of  it  as  a 
mydriatic,  meaning  that  it  not  only  dilated  the  pupil  but 
paralyzed  the  accommodation.  Since  then  we  have  a  con- 
siderable number  of  drugs  which  dilate  the  pupil  'without 
affecting  the  accommodation — at  least,  in  certain  doses, — so 
that  now  we  make  a  distinction  between  this  class,  the 
mydriatics,  and  the  cycloplegics  which  do  paralyze  the  accom- 
modation. 


FIG.  121. — Changes  in  the  range  of  acccommodation  and  in  the  diameter  of 
the  pupil  after  the  application  to  the  conjunctiva  of  "  a  drop"  of  a  solution  of 
belladonna  one  part  to  120  (Donders). 

The  effect  of  a  full  dose  of  belladonna  upon  the  eye  has 
long  been  known.  To  observe  its  effects,  let  us  suppose 
that  we  have  a  solution  of  one  part  to  120  of  water 
(B  260,  p.  584).  If  a  single  drop  of  this  be  placed  on  the 


Full  Dose  of  Atropin  Sulphate 


161 


conjunctiva,  we  find  that  certain  important  changes  take 
place  in  the  eye,  such  as  Bonders  had  shown  long  ago.  Fig. 
121.  In  this,  the  numbers  on  the  horizontal  line  indicate 
the  minutes  after  the  application  has  been  made;  those 
in  the  vertical  column  reading  upward  from  infinity  give 
the  accommodation  of  the  individual,  in  the  inch  meas- 
urement, while  those  reading  downward  show  in  milli- 
meters the  width  of  the  pupil.  It  will  be  seen  that  in  about 
twelve  to  fifteen  minutes  the  proximate  point  begins  to 
recede,  and  continues  to  d'o  so  gradually  but  without  inter- 
ruption for  about  an  hour.  At  the  end  of  that  time  the 
proximate  point  has  reached  almost  its  farthest  limit,  and  at 
the  end  of  ninety  minutes  the  near  point  corresponds  with 
the  far  point — that  is,  no  power  of  accommodation  remains. 
This  condition  persists  for  about  two  days.  Then  the  effect 


FIG.  122. — Changes  in  the  range  of  accommodation  and  in  the  diameter  of  the 
pupil  for  fourteen  days  after  the  instillation  of  belladonna  one  part  to  120 
(Bonders). 

of  the  drug  begins  to  disappear,  and  the  manner  in  which 
this  is  done  is  shown  in  Fig.  122,  also  from  Bonders.  In 
this,  the  numbers  in  the  center  represent  the  days  after  the 
application  has  been  made,  those  in  the  vertical  column 
being  the  same  as  in  the  previous  figure.  We  see  that 
between  the  second  and  the  fourth  day  the  effect  lessens 


162 


Full  Dose  of  Atropin  Sulphate 


decidedly.  The  curve  which  represents  the  proximate  point 
rises  very  rapidly  at  first,  and  by  the  end  of  the  fourteenth 
day  has  returned  to  its  original  position.  Other  phenomena 
are  also  observed  in  this  connection.  For  example,  a  slight 
change  may  take  place  in  the  position  of  the  far  point,  and  it  is 
found  that  convergence  brings  into  action  a  certain  amount 
of  accommodative  power  which  is  not  manifested  when  each 
eye  is  tested  separately,  but  these  matters  are  not  of  interest 
in  this  connection.  Nor  is  the  behavior  of  the  pupil  of 
special  importance  now,  although  we  can  see  from  these 
diagrams  what  changes  that  also  undergoes. 

The  foregoing  illustrations  of  the  immediate  and  of  the 
subsequent  effects  of  a  full  dose  of  belladonna  were  first 
given  about  half  a  century  ago.  Since  then  they  have 
been  copied  by  Landolt,  by  Norris  and  Oliver,  and  in  nearly 
all  the  larger  text-books  in  different  countries.  In  view  of 
the  fact  that  so  many  practitioners  make  use  of  solutions 
of  atropin  for  this  purpose  day  after  day,  it  is  rather 


•—  , 

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X, 

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0  1 

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53 

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s 

FIG.  123. — Immediate  effect  of  a  full  dose  of  atropin. 

surprising  that  there  exist  very  few  corroborative  experi- 
ments to  show  the  exactness  of  these  well-known  curves. 
In  attempting  to  verify  the  measurements,  it  has  been 


Full  Dose  of  Atropin  Sulphate 


163 


very  difficult  or  impossible  to  obtain  curves  which  are 
as  regular  as  those  given  by  Bonders.  Especially  is  this 
the  case  with  the  weaker  doses,  or  soon  after  any  dose 
begins  to  have  its  effect.  Thus,  in  measuring  the  point 
at  which  the  person  says  he  can  see  a  given  set  of 
test  types  with  distinctness,  there  is  a  variation  some- 
times at  one  position  and  sometimes  at  another,  so  that  if 
the  examiner  records  accurately  what  he  finds,  independently 


13 


FIG.  124. — Effect  for  nine  days  after  full  dose  of  atropin. 

of  what  he  thinks  he  ought  to  find  at  a  given  time,  my  own 
trials,  at  least,  prove  that  the  resulting  curve  shows  various 
irregularities.  It  appears  also  that  Snellen  had  a  somewhat 
similar  experience.  (B  390).  Figures  123  and  124  show  the 
immediate  and  also  the  subsequent  effects  of  o.oooi  of  a 
gram  of  atropin  sulphate  applied  to  the  conjunctiva.  This 
amount  of  the  salt  is  approximately  the  same  as  that  con- 
tained in  a  "drop"  of  a  solution  of  one  grain  to  an  ounce, 
understanding,  as  we  should,  the  decided  variation  in  the 

1  In  this,  and  in  all  similar  diagrams  of  this  group,  the  numbers  on  the  left, 
reading  upward,  represent  diopters  of  accommodation,  while  the  numbers 
reading  downward  represent  millimeters  to  show  the  diameter  of  the  pupil. 


164  Atropin  Sulphate  as  a  Mydriatic 

size  of  drops.     That  is,  if  we  estimate  roughly  500  drops  to 
a  fluid  ounce,  then   each  drop  of  such  a  solution  would 

.061; 

contain  about  — -  or  0.00013  gram. 
500 

It  is  important  to  observe  in  this  connection  that  we 
obtain  for  the  majority  of  eyes  about  the  same  curve,  no 
matter  whether  we  apply  to  the  conjunctiva  ooooi  gram 
or  0.0005  gram  or  any  dose  between  those  limits.  When 
the  dose  is  rather  stronger  than  0.0005  gram,  the  curve 
does  not  drop  much  sooner  or  more  rapidly,  but  is  longer 
before  it  begins  to  rise.  When  the  amount  used  is  de- 
cidedly less  than  o.oooi,  we  find  quite  frequently  that 
the  curve  showing  changes  in  the  accommodation  drops 
slowly,  or  with  irregularities,  although  the  effect  on  the 
pupil  is  still  quite  constant.  For  practical  purposes,  there- 
fore, we  seem  warranted  in  considering  any  amount  be- 
tween these  limits  as  what  may  be  called  a  "  full  dose." 
Several  of  the  manufacturers  prepare  an  ophthalmic  tablet 
containing  -%$$  of  a  grain,  and  also  -^^  of  a  grain. 

§  8.  What  is  the  Minimum  Amount  of  Atropin  Sul- 
phate which  will  Ordinarily  Dilate  the  Pupil  ?  — Inasmuch 
as  atropin  acts  both  as  a  mydriatic  and  as  a  cycloplegic,  and 
as  it  requires  a  decidedly  less  amount  to  dilate  the  pupil 
than  to  affect  the  accommodation,  it  is  desirable  to  con- 
sider first  the  minimum  dose  which  will  act  as  a  mydriatic, 
before  we  study  the  minimum  amount  which  will  paralyze 
the  accommodation,  although  for  clinical  purposes  the  latter 
is  much  the  more  important  aspect  of  the  question. 

Only  a  few  references  can  be  found  in  the  literature  to 
the  minimum  amount  which  will  act  as  a  mydriatic.  Don- 
ders  (B  378,  p.  31)  says  that  if  a  solution  of  I  to  120,000 
is  kept"  long  "  in  contact  with  the  cornea  it  will  dilate  the 
pupil.  Later  Feddersen  (B  380)  tried  the  effect  of  very 
weak  solutions  upon  76  different  persons.  Among  these  he 
found  that  when  he  applied  to  the  conjunctiva  enough  of  a 
solution  to  represent  o.oooooi  gram,  there  was  mydriasis  in 
about  42  %  of  these  76  persons,  and  when  he  used  0.000002 
gram  every  pupil  dilated.  My  own  experiments  on  the  eyes 
of  soldiers  accord  fairly  well  with  these  findings. 


Minimum  Dose  of  Atropin  Sulphate          165 

An  examination  of  the  statements  made  on  this  subject, 
even  by  trained  observers,  shows  that  considerable  confusion 
exists  as  to  the  use  of  the  term  "mydriasis."  Some  writers 
mean  by  this  word  a  slight  enlargement  of  the  pupil  and 
others  apply  it  only  to  the  maximum  dilatation.  We  shall 
see  later  that  complete  dilatation  of  the  pupil  is  often  as 
difficult  to  obtain  as  is  complete  relaxation  of  the  accom- 
modation. For  practical  purposes,  therefore,  we  may  con- 
sider that  the  pupil  of  an  adult  is  dilated  when  it  measures 
more  than  seven  or  eight  millimeters  in  diameter.  If,  there- 
fore, we  are  to  understand  by  "mydriasis"  not  simply  the 
slight  enlargement  of  the  pupil,  but  its  considerable  dilata- 
tion, then  apparently  it  is  better  to  place  the  minimum 
amount  of  atropin  sulphate  necessary  to  produce  this  at 
nearer  0.000005.  Even  a  very  much  smaller  amount  will 
produce  marked  dilatation  of  the  pupil  of  a  dog  or  other 
animal  whose  cornea  is  thin. 

§  9.  What  is  the  Minimum  Amount  of  Atropin  Sul- 
phate which  will  Ordinarily  Relax  the  Accommodation  ? 
— If  the  aim  of  the  practitioner  is  simply  to  put  the  ciliary 
muscle  at  rest — in  other  words,  if  he  only  cares  to  determine 
the  refraction  of  an  eye,  then  the  information  which  we  have 
long  possessed  as  to  the  physiological  effect  of  a  full  dose  is 
quite  sufficient.  But  if  it  is  desired  to  learn  also  something 
cf  the  condition  of  the  accommodation  in  a  given  case,  he  must 
use  a  small  amount  of  the  drug  and  then  observe  carefully 
the  behavior  of  the  ciliary  muscle.  When  we  come  to  deal 
with  the  pathological  aspects  of  this  study,  we  shall  learn 
that  an  imperfect  action  of  the  ciliary  muscle  is  one  of  the 
most  frequent  and  important  causes  of  muscle  imbalance. 
Moreover,  the  behavior  of  that  muscle  when  acted  upon 
by  small  doses  of  atropin  sulphate  is  often  of  considerable 
diagnostic  value  in  showing  whether  there  exists  a  tendency 
to  excessive  or  to  insufficient  contraction. 

This  question  as  to  the  minimum  dose  of  belladonna  neces- 
sary to  relax  the  accommodation  of  the  average  normal  eye 
did  not  escape  the  notice  of  Kuyper(B  360,  p.  587),  although 
it  is  not  clear  how  many  "drops"  were  used.  In  order  to 
obtain  at  least  a  few  data  bearing  on  the  point  I  have  used 


1 66          Minimum  Dose  of  Atropin  Sulphate 

atropin  sulphate  in  various  doses  upon  the  eyes  of  thirty-one 
soldiers  stationed  at  Fort  Porter,  Buffalo,  and  a  few  other 
younger  persons. 

The  eyes  of  each  of  these  men  had  been  carefully  tested 
on  entering  the  service.  Each  had  vision  equal  to  $  and 
the  ametropia  present  did  not  exceed  0.75.  The  men  had 
all  been  practically  free  from  asthenopic  symptoms,  and 
were  otherwise  in  good  health.  At  this  point  it  is  impos- 
sible to  do  more  than  to  state  very  briefly  that  the  results 
confirm  in  general  the  findings  of  Kuyper,  and  to  add  a 
summary  of  the  observations  concerning  the  behavior  of  the 
accommodation : 


>—_, 

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^ 

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X^ 

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S 

. 

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52 

02 

53 

03 

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56 

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0  7 

58 

08 

59 

09 

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0 

1 

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10 

2^ 

—"  "" 

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^ 

FIG.  125. — Immediate  effect  of  a  minimum  close  of  atropin  on  a  young 
Subject. 

1st.  'When  0.000005  to  o.ooooi  gram  (about  TiT¥inr  to 
•g-^ofl  grain)  of  atropin  sulphate  is  applied  to  the  conjunc- 
tiva, it  is  sufficient  ordinarily  to  produce  a  relaxation  of  the 
accommodation  (Fig.  125). 

2d.     Although    this  relaxation  begins  in  from  10  or  20 

*  Appreciation  should  also  be  expressed  of  the  co-operation  of  the  surgeons- 
stationed  at  Fort  Porter,  especially  Majors  Halleck  and  Kendall.  Without 
their  good  offices  and  their  explanations  to  the  soldiers,  the  men  would  not 
have  offered  themselves  as  willingly  as  they  did,  no  matter  what  induce- 
ments were  held  out  to  them. 


Minimum  Dose  of  Atropin  Sulphate          167 


to  25  minutes,  the  full  effect  is  not  reached  till  more  than 
an  hour  and  a  half  after  the  application. 


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FlG.  126. — Effect  four  and  a  half  days  after  a  minimum  dose  of  atropin. 

3d.  Within  a  few  hours  the  effect  begins  to  subside  and 
has  nearly  disappeared  by  the  second  day. 

4th.  The  curve  representing  these  changes  in  the  accom- 
modation varies  somewhat  in  different  persons  with  normal 
eyes,  this  depending  upon  the  susceptibility  of  the  indi- 
vidual as  a  whole  to  the  drug,  and  in  pathological  conditions 
upon  the  excessive  or  insufficient  contraction  of  the  ciliary 
muscle.  Apparently  the  age  of  the  individual  is  also  a 
factor  in  determining  the  prompt  or  tardy  action  of  this  and 
of  similar  drugs,  the  irides  of  young  persons  seeming  to 
respond  more  promptly  to  such  stimuli  than  do  those  of 
middle  or  later  life.  We  must  conclude,  therefore,  that  the 
amount  mentioned  may  be  considered,  in  general,  the  mini- 
mum dose  which  will  produce  distinct  relaxation  of  the 
ciliary  muscle.  It  happens  that  one  or  two  of  the  manufac- 
turers make  an  ophthalmic  tablet  containing  y^T)  °f  a  grain- 
One  of  these  or  a  half  of  one  therefore  represents  a  minimum 
dose. 

It  might  perhaps  be  asked,  does  this  amount  of  atropin 


1 68  Value  of  Minimum  Doses 

sulphate  produce  always  the  curves  shown  above?  The 
answer  to  this,  as  a  general  proposition,  evidently  must 
be  in  the  negative.  As  with  any  amount  much  less  than 
the  full  dose  the  resultant  curve  tends  to  vary  from  the 
types  given,  (Figs.  120  to  124),  so  are  there  many  slight 
variations  from  the  curve  furnished  by  this  minimum  dose. 
Indeed,  it  is  probable  that  no  two  eyes  would  give  always 
exactly  the  same  curve. 

The  individual  variations  in  the  forms  of  the  curves  may 
be  due  to  two  causes.  One  of  these  we  call  the  "  suscepti- 
bility" of  the  individual  to  the  drug,  and  another  depends 
upon  the  condition  of  the  ciliary  muscle  of  the  eye  itself. 
The  real  fact  is,  however,  that  when  we  make  tests  with 
small  amounts  of  atropin  upon  normal  eyes,  although,  as 
just  stated,  the  details  of  these  curves  may  vary  somewhat, 
still,  the  gradual  fall  in  the  curve  is  so  characteristic  as  to  be 
recognized  almost  immediately. 

It  should  be  observed  also  that  the  susceptibility  of  the 
individual  to  the  drug  is  in  reality  not  a  factor  by  any 
means  as  important  as  might  be  imagined.  It  is  worthy 
of  note  in  this  connection  that  although  some  practitioners 
are  accustomed  to  use  strong  solutions  of  atropin,  and  often 
very  carelessly  in  office  work,  still  it  is  quite  rarely  that  we 
see  the  constitutional  symptoms  of  belladonna. 

§  10.  What  is  the  Practical  Value  of  Minimum  Doses 
of  Atropin  Sulphate? — They  assist  us  in  deciding  whether 
the  ciliary  muscle  is  in  a  normal  condition  or  whether  it 
relaxes  more  or  less  readily  than  it  should.  In  doing  this  we 
apply  a  given  amount  to  the  eye  and  then  notice  what 
the  effect  is  upon  the  accommodation,  and  incidentally  upon 
the  size  of  the  pupil,  observing  both  the  time  and  the  extent 
of  the  effect  produced. 

Let  us  suppose,  for  example,  that  we  use  a  disc  containing 
0.000005  gram  of  atropin.  The  curve  for  this  as  it  affects  the 
accommodation  has  already  been  ascertained.  If  now  we  use 
the  same  upon  an  asthenopic  eye,  and  find  that  it  takes  a 
longer  time  for  the  relaxation  of  the  accommodation  to  take 
place  that  indicates  that  the  ciliary  muscle  is  in  a  state  of 
abnormal  contraction. 


Homatropin 


169 


Or  it  may  happen  that  after  the  application  the  line  indi- 
cating the  relaxation,  instead  of  being  a  curve,  seems  to  drop 
suddenly  and  to  an  unusual  extent.  It  is  then  fair  to  infer, 
other  things  being  equal,  that  the  ciliary  muscle  is  more 
easily  affected  than  in  the  normal  condition,  unless  of  course 
the  individual  be  unusually  susceptible  to  belladonna. 

§  ii.  The  Effect  of  a  Full  Dose  of  Homatropin. — It 
would  be  interesting  in  this  connection  to  observe  the  effects 
of  various  drugs  which  have  been  used  as  cycloplegics,  but 
that  would  necessitate  too  long  a  digression.  It  is  worth 
while,  however,  to  enquire  as  to  the  physiological  effect  of 
homatropin.  This  is  Used  so  constantly,  especially  in  America, 
as  a  cycloplegic  and  also  as  a  mydriatic,  that  we  might  expect 
to  find  many  exact  measurements  had  been  made  to  deter- 
mine its  physiological  action.  Such,  however,  is  not  the 
case.  Of  the  very  few  which  are  recorded,  perhaps  the  most 
accurate  are  those  of  Straub  (B.  397). 


13 
12 
1  1 
10 
9 
8 
7 
6 
5 
4 
3 
2 
;  1 

f 

2 
3 

a 

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^ 

-^ 

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1 

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02 

53 

03 

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01 

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56 

0  6 

5  7 

0  7 

58 

0  8 

59 

09 

5  1 

0 

1! 

0 

i: 

0 

1 

JO 

1' 

0 

5 
6 
7 
8 

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1 

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FIG.  127. — Immediate  effect  of  a  full  dose  of  homatropin. 

But,  as  we  ordinarily  employ  rather  larger  doses  of  the 
drug  for  clinical  purposes  than  he  used,  it  seemed  worth 
while  to  note  the  effect  of  these  upon  the  normal  ciliary 
muscle.  Figure  127  shows  the  effect  immediately  after  the 


Homatropin 


application  of  a  disc  containing  -fa  of  a  grain  (0.0013  gram)  of 
homatropin  hydrobromate  to  the  eye  of  a  boy  fifteen  years 
old.  From  this  curve  it  appears  that  the  decrease  in  the 
range  of  accommodation  begins  in  about  ten  minutes,  and 
it  is  effaced  decidedly  sooner  than  with  what  we  may  call 
a  full  dose  of  atropin  sulphate. 


10 


13 


FIG.  128. — Effect  ten  hours  after  a  full  dose  of  homatropin. 

But  the  important  point  is  of  course  that  complete  relaxation 
of  the  accommodation  lasts  a  comparatively  short  time  (Fig. 
128).  By  the  end  of  the  third  hour  it  is  again  apparent,  and 
this  increases  so  rapidly  that  by  the  end  of  the  tenth  hour 
after  the  application  has  been  made  the  normal  condition  in 
this  respect  is  almost  restored.  It  should  be  mentioned, 
however,  that  in  a  considerable  number  of  individuals,  after 
the  application  of  the  same  dose,  a  slight  imperfection  in  the 
accommodation  continues  even  until  the  third  or  fourth  day. 
The  dilatation  of  the  pupil  keeps  pace  with  the  changes  in 
the  accommodation,  as  will  be  seen  by  a  glance  at  the 
curves.  If  the  dose  is  much  smaller  than  this,  although  the 
accommodation  may  relax  promptly,  that  effect  begins  to 
disappear  almost  at  once. 

It  should  be  remembered  that,  when    either   atropin  or 


Atropin  or  Homatropin  ?  171 

homatropin  is  used  for  clinical  purposes,  it  is  unnecessary  to 
have  the  patient  suffer  the  resulting  inconvenience  for  as 
long  a  time  as  these  curves  would  indicate.  Instead  of  that, 
most  practitioners  then  apply  to  the  eye  a  small  amount  of 
eserin.  We  do  not  yet  know  what  amount  of  this  will 
counteract  the  effect  of  a  given  dose  of  atropin  or  homatropin. 
It  is  possible  also  that  this  double  effect  of  drugs  in  an  eye 
is  a  disadvantage  to  it.  Any  one  who  has  tried  this  experi- 
ment on  himself  with  rather  strong  doses  can  testify  that  the 
effect  is,  under  some  circumstances,  exceedingly  disagreeable. 
It  is  certain,  however,  that  with  a  small  dose  of  eserin  it  is 
possible  to  cut  short  very  decidedly  the  effect  of  any  of  these 
cycloplegics,  and  if  the  dose  G;  the  rriyotic  is  small,  the  result- 
ing discomfort  is  comparatively  slight. 

§  12.  What  is  the  Diagnostic  Value  of  Atropin 
Sulphate  as  Compared  with  Homatropin  Hydrobro- 
mate  ? — In  order  to  answer  this  question,  let  us  under- 
stand first  what  the  desiderata  are  for  a  cycloplegic,  and 
then  determine  how  well  one  or  the  other  fulfils  the  condi- 
tions. It  is  generally  admitted  that  we  desire  a  drug — • 

1st.     Which  is  safe. 

2d.  Which,  under  normal  conditions,  does  really  relax 
the  accommodation. 

3d.  Whose  effects  last  long  enough  to  make  it  rea- 
sonably certain  that  the  examination  of  the  refraction 
can  be  made  during  the  period  of  relaxation. 

4th.  Whose  disagreeable  effects  pass  off  in  the  minimum 
time. 

5th.     Which  is  inexpensive. 

ist.  As  to  the  question  of  safety,  although  atropin  sul- 
phate as  used  for  this  purpose  cannot  be  considered 
dangerous,  still  persons  who  have  an  idiosyncrasy  in  this 
respect  are  met  with  occasionally,  and  its  effects  are  then  so 
annoying  that  it  must  be  admitted  that  homatropin  hydro- 
bromate  is  preferable  from  this  point  of  view. 

2d.  Atropin  sulphate,  even  in  minimum  quantity,  is 
apparently  much  more  constant  in  its  action  upon  normal 
eyes  than  homatropin  hydrobromate. 

3d.     The   effect  of  atropin  persists  for  a  much  longer  time 


172  Atropin  or  Homatropin  ? 

than  does  that  of  homatropin..  This  renders  it  more  cer- 
tain that  the  tests  are  made  when  the  accommodation  is 
most  relaxed,  provided  there  be  no  fault  in  the  accommoda- 
tion. 

4th.  On  the  other  hand,  atropin  has  the  disadvantage 
that  it  gives  a  corresponding  amount  of  inconvenience  to  the 
patient.  When  we  remember,  however,  that  the  main  object 
in  the  use  of  any  such  drug  is  usually  to  ascertain  the  condi- 
tion of  the  refraction,  it  is  apparently  better  to  suffer  annoy- 
ance from  the  drug  a  little  longer,  than  run  the  risk  of  having 
its  effect  insufficient.  The  fact  is  also,  as  just  noted,  that  the 
effects  of  atropin  and  especially  of  homatropin  can  be  coun- 
teracted somewhat  by  the  use  of  a  small  amount  of  eserin. 
When  we  come  to  consider  cases  in  which  there  is  abnormal 
contraction  of  the  ciliary  muscle,  we  shall  see  that,  if  any 
spasm  be  present,  a  dose  of  homatropin  which  will  entirely 
relax  the  accommodation  of  the  normal  eye  is  then  in- 
sufficient. 

5th.  As  to  economy,  atropin  sulphate  is  of  course  much 
the  cheaper.  While  this  is  a  matter  of  minor  importance 
for  many  who  come  to  a  private  office,  it  is  not  to  be 
forgotten,  especially  when  a  drug  is  to  be  bought  in 
considerable  quantities  for  an  institution.  We  may  there- 
fore conclude : 

1st.  To  determine  the  refraction  as  accurately  as  is  possi- 
ble at  a  single  visit,  it  is  best  to  use  atropin  about  o.OOOi  to 
0.00026  gram  (a  disc  of  5-^¥  or  -^^  grain). 

2d.  To  learn  the  condition  of  the  accommodation,  we 
should  use  atropin  about  0.000005  to  o.ooooi  gram  (a  disc 
of  •gVinj'  Sram)  or  a  corresponding  part  of  it. 

3d.  When  we  are  satisfied  to  know,  perhaps  approxi- 
mately, the  condition  of  the  refraction  without  regard  to  the 
accommodation,  about  0.0013  gram(-g*g-  grain)  of  homa- 
tropin is  by  far  the  most  convenient. 

4th.  Asa  mydriatic,  homatropin  is  infinitely  the  better 
of  the  two.  For  that  purpose  a  disc  of  o.oooi  gram  (-$$-$ 
grain)  is  usually  sufficient. 

§  13.  Cocain  as  a  Mydriatic  and  a  Cycloplegic. —  In 
this  connection,  some  mention  should  be  made  of  the  effect 


Cocain  173 

of  cocain.  Its  action  upon  the  intraocular  muscles  has 
been  measured  by  Straub  (B  397,  p.  216)  and  others,  and 
the  results  are  of  course  easily  verified.  Let  us  consider  it : 

(A)  As  a   mydriatic.       When    about    0.0013    gram  ( ^ 
grain)  is  applied  to  the  conjunctiva,  the  pupil  dilates  within 
fifteen    minutes,     and  this    continues  slowly     for  an  hour. 
It  remains  at  this  maximum  size  for  about  half  an  hour,  and 
then   begins  to  contract.     At  first  this  contraction  is  quite 
rapid,  then  more  gradual,  and  the  effect  disappears  entirely 
after  about  twelve   hours.     It  is  interesting  to  notice  that 
this  dose  is  not  sufficient  to  produce  complete  mydriasis, 
as    the    pupil  still   contracts    somewhat  in    a   bright  light. 
While,  therefore,    cocain    cannot   be    considered  a  reliable 
mydriatic  in  doses  of  this  size,  it  is  nevertheless  very  con- 
venient when  only  an  enlargement  of  the  pupil  is  desired. 

(B)  Asacycloplegic.    This  less  conspicuous  action  is  often 
overlooked.     A  good  illustration  of  this  effect  is  shown  in 
the  subject  of  experiment  by  Straub.      In  that  person,  after 
the  application  of  about  o.ooi  gram  the  accommodation  fell 
from  6.5  D.  to  about  4. 5  D.  within  half  an  hour.    It  remained 
at  that   point  for   nearly  an    hour   and    a   half,  and  then 
gradually  rose  to  its  normal  point  at  the  end  of  six  hours. 
While  the  accommodation  is  not  always  affected  in  this  way, 
even  by  considerable  doses,  there  is  no  question  that  this 
result  is  noticeable  in  certain  cases.     The  point  is,  that  if 
we  use  ophthalmic  discs  or  solutions  of  various  drugs  which 
contain  also  a  considerable  proportion  of  cocain,  we  should 
remember  that  the  cycloplegic  effect  may  be  due  in  part  to 
the  cocain  or  to  the  combined  effect  of  the  drugs. 

(C)  Mention   may  also  be  made  here  of  the  minimum 
amount   of  cocain   necessary   to  produce   complete   anaes- 
thesia, because  this  drug  is  often  used  in  haphazard  fashion 
and  in  larger  quantities  than  is  necessary.     Soon  after  it 
came  into  use,  a  series  of  experiments  made  in  the  laboratory 
of   the  Landwirtschaftliche   Hochschule  in    Berlin    gave   a 
numerical   expression    of   the   smallest   dose   which   would 
produce  the  maximum  amount  of  anaesthesia  (B  400).   These 
experiments  were  based  on  the  well-known  physiological  fact 
that  irritation  of  a  sensitive  nerve  produces  a  rise  in  the 


Cocain  for  Anaesthesia 


blood  pressure  practically  in  proportion  to  the  degree  of 
the  irritation.  Accurate  measurements  of  the  effect  of 
an  irritant  (in  the  form  of  an  electric  current  of  given 
strength),  when  applied  to  the  normal  eye  and  also  to  an 
eye  after  the  application  of  cocain  in  various  amounts, 
showed  that  complete  anaesthesia  was  produced  in  rabbits 
not  until  about  fifteen  minutes  after  the  application  of  the 
full  dose.  This  fact  is  of  importance  in  connection  with  the 
effect  of  the  drug  on  the  intraocular  muscles,  and  it  also 
gives  an  indication  as  to  its  use  in  the  various  operations  on 
the  muscles,  which  will  be  considered  later. 

§  14.  The  Effect  of  a  Full  Dose  of  Eserin.— Calabar 
bean  was  studied  by  Bonders  (B  260),  and  his  curves  show- 
ing the  effects  of  what  he  calls"  a  sufficient  dose  "  are  usually 
given  in  the  larger  text-books.  In  order  to  verify  them  and 


0  \S  20253035 


0  5560  65  70  7583659095H 0 


FIG.  129. — Immediate  effect  of  a  full  dose  of  eserin. 


to  obtain  a  curve  resulting  from  a  given  amount,  a  disc  of 
^i^  of  a  grain  (o.ooor  gram)  was  applied  to  a  normal  eye. 
The  effect  is  seen  in  Figs.  129  and  130.  From  these  it  ap- 


Full  Dose  of  Eserin 


175 


pears  that  the  drug  affects  both  the  near  and  the  far  point. 
The  former  commences  to  approach  within  ten  to  fifteen 
minutes  after  the  application,  and  continues  to  come  nearer 


; 

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41 

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11 

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fi 

FIG.  130. — Effect  three  days  after  a  full  dose  of  eserin. 

to  the  eye  for  a  little  over  half  an  hour.  Then,  however,  it 
recedes  quite  rapidly.  By  the  end  of  an  hour  the  effect  has 
begun  to  subside,  by  the  end  of  six  hours  this  near  point  is 
still  farther  off,  and  the  eye  has  resumed  its  normal  con- 
dition toward  the  end  of  about  the  third  day. 

The  far  point  is  influenced  in  a  similar  way.  It  begins  to 
approach  within  ten  or  fifteen  minutes,  at  first  quite  rapidly, 
coming  nearest  to  the  eye  in  a  little  more  than  half  an 
hour.  The  far  point  then  recedes  and  within  the  first  day 
the  effect  upon  it  has  almost  entirely  disappeared. 

The  effect  of  a  full  dose  of  eserin  upon  the  pupil  is  char- 
acteristic and  is  seen  in  the  lower  part  of  the  same  figures. 
The  contraction  begins  about  the  same  time  that  the 
accommodation  is  affected  and  advances  constantly  and 


1 76 


Full  Dose  of  Eserin 


rapidly  for  the  first  fifteen  to  twenty  minutes,  reaching  the 
maximum  point  usually  in  about  half  an  hour.  The  pupil 
remains  in  that  condition  for  about  two  hours,  then  begins 
to  dilate,  rather  slowly  at  first.  By  the  end  of  five  or  six 
hours  the  change  is  very  apparent,  but  the  pupil  does  not 
return  to  its  original  size  until  the  expiration  of  the  second 
or  third  day. 

Although  this  amount  of  eserin  produces  its  effect 
promptly  and  completely,  it  should  be  stated  that  a  full 
dose  may  range  from  about  0.00006  to  0.0005  gram.  The 
curve  produced  by  one  of  the  larger  doses  is  similar  to  that 
just  given,  except  that  it  rises  a  little  more  abruptly,  and 
the  full  effect  may  be  maintained  for  several  days  longer. 


13 
12 
11 
10 
9 
8 
7 
6 
5 
4 
3 
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FIG.  131. — Immediate  effect  of  a  minimum  dose  of  eserin. 

§  15.  What  is  the  Minimum  Dose  of  Eserin  Suitable 
for  Diagnostic  Purposes  ? — A  considerable  number  of 
trials  with  weak  doses  of  eserin  have  shown  that  for  the  aver- 
age normal  eye,  about  0.000005  to  o.ooooi  gram  is  the  mini- 
mum amount  which  can  ordinarily  be  relied  upon  to  pro- 
duce the  characteristic  effect.  This  is  in  general  similar 
to  that  of  a  full  dose,  except  that  it  is  rather  longer  before 
the  effect  is  apparent  and  its  disappearance  is  more  rapid. 

When  we  examine  the  curve  more  in  detail  we  find,  first, 


Minimum  Dose  of  Eserin 


177 


that  the  near  point  rises  with  some  irregularity  and  slowly 
during  the  first  hour,  and  soon  after  reaching  the  maximum 
point  begins  to  subside. 


FIG.  132. — Effect  two  days  after  a  minimum  dose  of  eserin. 

The  far  point  follows,  in  general,  the  same  course.  Within 
an  hour  it  has  reached  its  nearest  point,  then  it  likewise 
begins  to  recede,  and  by  the  end  of  five  or  six  hours  is  prac- 
tically at  the  same  point  as  before  the  application  was  made. 

The  effect  on  the  pupil  of  this  quantity  is  similar  in  charac- 
ter to  that  of  a  full  dose,  but  less  in  degree.  Contraction 
begins  in  about  fifteen  to  twenty  minutes  and  reaches  the 
maximum  point  in  ten  or  fifteen  minutes  more,  and  although 
dilatation  then  begins,  the  pupil  does  not  return  to  its  original 
size  for  twelve  or  twenty-four  hours. 

§  1 6.  The  Clinical  Value  of  a  Minimum  Dose  of  Eserin 
is  Similar  to  that  of  Atropin. — When  a  certain  amount  of 
either  is  applied  to  the  conjunctiva,  we  can  judge  at  least 
approximately  whether  the  effect  upon  the  accommodation 
and  upon  the  pupil  is  of  a  greater  or  less  degree  than  in  the 
normal  eye.  Thus  a  myotic  may  give  evidence  corroborating 
that  which  is  furnished  by  the  cycloplegic.  References  will 
be  made  to  this  point  again  in  the  consideration  of  the  forms 


178  Clinical  Use  of  Minimum  Doses 

of    muscle    imbalance    which    depend    upon    excessive    or 
insufficient  action  of  the  ciliary  muscle. 

§  17.  What  is  the  Clinical  Method  of  Determining 
the  Effect  of  Minimum  Doses  of  Cycloplegics  and 
Myotics  ? — As  we  should  keep  constantly  in  view  the 
practical  application  of  these  various  physiological  facts,  the 
ophthalmologist  may  ask  how  it  is  possible  for  a  busy  practi- 
tioner to  give  all  the  time  necessary  to  measure  the  effect  of 
any  cycloplegic  or  myotic.  Or,  even  if  that  is  possible,  the 
question  also  arises  whether  «uch  frequent  measurements  are 
necessary  for  practical  purposes.  Fortunately  a  negative 
reply  can  be  given  to  both  of  these  questions.  In  rare  cases 
where  the  complexity  and  persistence  of  obstinate  symptoms 
render  the  utmost  exactness  desirable,  undoubtedly  it  is 
much  better  to  make  the  measurements  frequently.  In 
ordinary  cases,  however,  that  is  by  no  means  necessary. 
With  them,  after  measuring  the  accommodation  and  refraction 
in  the  usual  way,  after  determining  the  nearest  point  at 
which  the  smallest  type  can  be  seen,  and  after  applying  a 
disc  containing  a  minimum  dose,  as  already  indicated,  then 
it  is  sufficient  in  about  twenty  minutes  to  measure  the  near 
point,  again  in  another  twenty  minutes  a  second  time,  and  if 
the  changes  in  the  distance  of  the  near  point  seem  unusual, 
then  after  the  end  of  a  third  twenty  minutes  still  a  third 
measurement  can  be  made.  This  gives  at  least  a  general 
idea  as  to  the  rapidity  with  which  the  drug  acts.  If  the 
blanks  already  mentioned  are  at  hand,  the  record  can  be 
made  graphically  with  a  single  mark  of  the  pencil.  Such 
data  of  course  are  not  accurate,  but  they  are  sufficient  at 
least  to  corroborate  evidence  obtained  from  other  sources, 
and  serve  in  part  to  confirm  a  diagnosis  of  insufficient  or 
of  excessive  accommodation  when  either  of  these  exists. 


CHAPTER   III. 
ONE   EYE   IN   MOTION. 

§  i.  Nomenclature. — First,  it  is  necessary  to  agree  on 
the  definition  of  our  terms  expressing  motion  of  the  globe,  for 
different  meanings  have  been  given  them  by  different  writers. 
The  following  relate  to  movements  of  the  normal  as  well  as 
the  abnormal  eye,  but  the  terms  which  describe  pathological 
positions  and  movements  only  will  be  considered  later. 

At  present  we  have  to  do  with  adduction,  when  the 
cornea  is  turned  toward  the  median  line ;  abduction,  toward 
the  temple;  superduction,  straight  upward;  subduction, 
straight  downward.  Circumduction  or  cycloduction  is  a  form 
of  torsion, — that  is,  a  wheel-like  motion  of  the  eye  around 
the  optic  or  the  visual  axis ;  intorsion  is  the  rotation  of  the 
upper  end  of  the  vertical  axis  toward  the  median  line; 
extorsion,  the  rotation  of  the  upper  end  of  that  axis  toward 
the  temple.  We  may  have  also  a  true  torsion,  when  the 
globe  actually  makes  this  wheel  movement,  either  in  or  out, 
or  false  torsion,  when  the  wheel  movement  is  the  result  of 
rotation  about  some  other  than  the  optic  axis.  From 
these  nouns,  verbs  have  been  made — to  adduct,  to  intort, 
to  extort,  etc. 

A  protest  should  be  made  against  the  use  of  some 
of  these  words  which  have  been  brought  into  our  English 
terminology,  and  have  no  more  right  to  existence  than  the 
barbarism  "  to  refract,"  and  other  words  of  that  sort.  The 
fact  is,  however,  that  they  are  sanctioned  by  usage  in  this 
country  and  in  England,  and  rather  than  add  to  the 

179 


I  So  Ophthalmotropes 

confusion  by  introducing  new  terms  it  is  better  usually  to 
accept  the  old,  even  if  it  must  be  done  with  a  protest.  If 
any  change  in  the  general  nomenclature  is  made,  it  should 
be  by  the  action  of  national  or  international  ophthalmo- 
logical  societies,  as  Duane  has  suggested.  Two  other  terms 
borrowed  from  astronomical  terminology  should  be  added 
to  this  list.  They  assume  that  the  horizontal  plane  of  the 
eye  can  be  compared  to  the  actual  horizon,  and  the  vertical 
meridian  to  that  from  which  the  degrees  in  longitude  are 
reckoned,  as  from  Greenwich.  Understanding  this,  we  may 
speak  of  azimuth,  the  distance  in  degrees  in  or  out  from  the 
vertical  meridian. 

Plus  azimutJi  is  to  the  right  from  the  center  of  the  cornea 
when  that  is  in  the  primary  position,  and  minus  azimuth  to 
the  left.  Altitude  is  distance  in  degrees  above  the  hori- 
zontal plane  and  is  marked  plus.  Declination  is  the  distance 
below  the  horizontal  plane  and  is  marked  minus. 

With  this  system,  any  direction  from  the  center  of  the 
cornea  can  be  described  and  located. 

Thus,  to  indicate  that  the  center  of  the  cornea  had  been 
turned  to  the  right  15  degrees  and  downward  20  degrees, 
we  would  write  -{-15  —  20,  the  example  of  the  astronomers 
being  followed  again  in  giving  the  azimuth  first  and  altitude 
or  declination  afterwards.  It  is  understood  that  the  globe 
does  not  necessarily  turn  first  fifteen  degrees  to  the  right 
and  then  twenty  degrees  down,  but  rotates  about  an  oblique 
axis  lying  in  Listing's  plane,  the  center  of  the  cornea  passing 
from  the  primary  position  straight  to  the  spot  indicated. 

§  2.  Ophthalmotropes. — In  an  article  by  Bonders  in 
1870  (B  416),  he  says:  "  In  spite  of  the  fact  that  our  knowl- 
edge of  the  motions  of  the  eyes  is  already  fairly  complete, 
the  subject  remains  a  stumbling-block  to  many  ophthal- 
mologists. The  literature  contains  many  contradictions, 
especially  in  regard  to  the  so-called  wheel  motion,  and 
lecturers  frequently  see  the  earnest  attempts  of  the  listeners 
to  obtain  a  clear  appreciation  of  this  mechanism  end  in  dis- 
astrous failure.  For  this  reason  attempts  have  been  made 
to  produce  mechanical  representations  which  assist  in  giving 
a  clear  idea  of  the  subject,  and  these  instruments  have  been 


Landolt's  Ophthalmotrope 


181 


called  Ophthalmotropes."  This  statement  still  holds  true, 
unfortunately,  to  such  an  extent  that  we  also  must  ask  what 
assistance  ophthalmotropes  give  us  to-day . 

I.  The  plain  rubber  ball.     The   simplest  variety  of  the 
ophthalmotrope,  and  one  which  has  been  invented  by  many 
a  student,  consists  of  nothing  more  than  a  rubber  ball  trans- 
fixed   by    three    knitting    needles    representing  the    three 
principal    axes, — namely,  the  optic,  the    vertical,    and    the 
transverse.     This  model  is  one  of  the  most  useful. 

II.  Landolt's    ophthalmotrope.       The    rubber   ball    was 
improved  by  Landolt  (6424)  who  marked  upon  it  the  vertical 
and  horizontal  meridians,  and  the  anterior  extremity  of  the 
axes  of  rotation  of  the  two  oblique  muscles  (O)  and  the  two 
vertical  recti  (R).     The  circle  described  on  the  ball  about 
the  point  of  O,  with  a  radius  from  O  to  the  center  of  the 
cornea,  would  indicate,  for  example,  the  path  which  the  cen- 
ter of  the  cornea  would  follow  if  the  globe  were  rotated  only 
by  the  oblique  muscles.     Or,  a  circle  described  on  the  ball 
about  the  point  R,  with  a  radius  from  R  to  the  center  of  the 


FIG.  133. — Rubber-ball  ophthalmotrope  with  markings  suggested  by  Landolt. 

cornea,  would  indicate  the  path  which  the  center  of  the 
cornea  would  follow  if  the  globe  were  moved  only  by  the 
superior  and  inferior  recti.  We  shall -see,  however,  that 


182 


Landolt's  Ophthalmotrope 


when  the  globe  moves  from  the  primary  to  any  secondary 
position  it  rotates  about  some  axis  in  Listing's  plane,  and 
as  the  axis  of  rotation  of  the  two  oblique  muscles,  and  also 
the  axis  of  the  superior  and  inferior  recti  do  not  lie  quite  in 
that  plane,  a  movement  about  those  axes  can  occur  only  un- 
der unusual  circumstances.  Consequently,  these  arcs  drawn 
on  Landolt's  rubber  model  are  apt  to  give  rise  to  confusion 
unless  this  point  is  appreciated.  (Fig.  133.) 

Later,  Landolt  sug- 
gested another  form  of 
an  ophthalmotrope  (B 
425),  which,  as  he  says, 
"  is  to  demonstrate  the 
direction  and  position 
which  the  eye  takes 
under  the  influence  of 
each  of  its  muscles.  A 
schematic  eye,  repre- 
sented in  outline  mere- 
ly by  bands  of  metal  for 
its  vertical  and  horizon- 
tal meridians  and  its 
equator,  with  a  cornea 
attached,  is  so  arranged 
as  to  be  suspended  in 
two  stationary  rings,  one 
horizontal  and  the  other 
vertical,  the  forms  of 
support  being  the  rotary 
axes  of  the  muscles." 
The  general  plan  of  this  model  is  shown  in  the  illustration 
here  given  (Fig.  134). 

The  arrangement  is  ingenious  and  in  many  respects  very 
convenient.  But  it  has  two  defects.  The  first  is  that  it 
illustrates  only  the  rotation  which  takes  place  around  the 
axes  of  the  horizontal,  the  vertical,  and  the  oblique  pairs  of 
muscles,  and  like  most  other  ophthalmotropes,  it  requires 
the  student  to  depend  entirely  on  his  imagination  to  locate 
the  position  of  the  retinal  image. 


FIG.   134. —  Landolt's   more  complete 
ophthalmotrope. 


Ophthalmotrope  of  the  Author  183 

III.  Bonders'  ophthalmotrope.     The  ophthalmotrope  of 
Donders  (B  416),  which  has  been  already  referred  to,  is  quite 
different  from  those  of   Landolt,  being  constructed   in  the 
form  of  an  eye  and  made  to  turn  in  a  series  of  rings  as  a 
compass  is  hung  in  position.     Apparently  it  is  not  manufac- 
tured now. 

IV.  Knapp's  ophthalmotrope  (B  257,  p.  667)  is  seen  in 
"Fig-  135  and  requires  no  extended  description.     The  muscles 
are  represented  by  strings,  each  one  being  attached  pos- 
teriorly to  a  small  weight.      When  the  eye  moves,  it  is  easy 
to  observe  by  the  changes  in  position  of  these  weights  just 
which  muscles  produce  that  rotation. 


FlG.  135. — Knapp's  ophthalmotrope. 

V.  The  ophthalmotrope  of  the  author.  In  as  much  as 
none  of  these  models  was  quite  satisfactory,  it  seemed  worth 
while  to  construct  yet  another.  Accordingly  I  have  had  one 
made  such  as  is  seen  in  Fig.  136.  A  sheet  of  brass  20  cm.  high 
by  30  cm.  long  is  perforated  by  two  circular  openings,  each 
6  cm.  in  diameter.  This  fixed  vertical  plane,  or,  more 
exactly,  the  flange  attached  to  it,  corresponds  to  Listing's 


1 84 


Ophthalmotrope  of  the  Author 


plane.     Around  the  posterior  edge  of  each  opening  there  is  a 
flange,  8  mm.  thick  and  12  cm.  in  diameter. 

The  edge  of  the  flange  is  graduated,  and  is  perforated  by 
16  tubes  or  canals,  4  mm.  in  diameter,  also  in  Listing's  plane, 


-1 


FIG.  136. — Ophthalmotrope  of  the  author. 

each  being  a  radius  from  the  center  of  the  eye.  They  are 
arranged  in  pairs  opposite  each  other,  and  if  continued 
would  constitute  eight  axes  of  the  globe  in  Listing's  plane. 
Into  each  of  these  canals  a  pin  can  be  inserted  and  so 
held  in  place  by  a  spring  that  it  tends  to  press  against  the 
globe,  thus  holding  the  latter  firmly  in  position.  It  is 
evident  that  any  two  pins  wrhich  are  opposite  each  other 
may  suspend  the  globe,  and  that  when  doing  so  they  repre- 
sent an  axis  about  which  the  globe  may  revolve  in  any  one 
of  eight  different  directions. 

The  part  of  the  Ophthalmotrope  which  represents  the  eye 
itself  is  a  hollow  globe  of  brass  5  cm.  in  diameter.  It  has 
an  opening  in  front  corresponding  to  the  pupil,  and  a  lens 
within,  of  sufficient  strength  to  bring  parallel  rays  to  a  focus 
on  a  disc  of  ground  glass  behind.  About  the  equator  of  the 
globe  there  is  a  band  4  mm.  wide  perforated  at  intervals 
by  small  openings  to  receive  the  ends  of  the  radial  pins. 
This  band  is  graduated,  and  has  attached  to  it  another 
graduated  arc  which  can  be  slid  into  any  position  desired. 
The  muscles  are  represented  by  strings.  At  proper  points 


these  strings  emerge  from  the  interior  and  pass  backward, 
to  be  attached  to  weights,  as  in  the  ophthalmotrope  of 
Knnpp. 

In  using  this  ophthalmotrope,  if  we  wish  to  observe  what 
occurs  when  the  eye  moves  directly  upwards,  we  place  the 
two  radial  pins  in  the  openings  of  the  flange  which  are  hori- 
zontal. As  the  globe  moves  upward  it  does  so  by  rotation 
about  an  axis  in  Listing's  plane.  We  can  then  see,  by 
changes  in  the  position  of  the  weights,  which  muscles  cause 
this;  at  the  same  time  the  degree  of  rotation  is  registered  by 
the  graduation  on  the  vertical  meridian  of  the  globe,  and  the 
position  of  the  retinal  image  on  the  ground  glass  plate  can  also 
be  easily  seen.  If  we  wish  to  observe  what  occurs  when  the  eye 
rotates  about  a  vertical  axis,  the  procedure  is  equally  simple. 
Or  again,  we  may  observe  what  occurs  when  the  globe 
revolves  about  an  oblique  axis — for  example,  an  axis  which 
passes  from  above  and  inward,  down  and  outwards.  To  do 
this,  two  pins  are  passed  through  the  openings  in  the  flange 
corresponding  to  that  position  ;  as  the  globe  rotates  about 
that  axis,  we  not  only  see  which  muscles  produce  that  change, 
but  we  can  measure  the  degree  of  the  revolution  by  the 
graduation  on  the  vertical  and  horizontal  meridians  and  see 
on  the  ground-glass  the  position  which  the  images  occupy. 

By  drawing  a  cross  with  two  fine  lines  on  the  ground-glass 
plate,  it  is  possible  when  using  proper  test  objects  to  see 
how  "  false "  torsion  is  produced  when  the  globe  rotates 
about  an  oblique  axis. 

The  advantages  of  this  model  are  : 

First.  It  represents  the  rotation  which  the  globe  actually 
makes,  that  being  always  about  an  axis  which  lies  in  Listings 
plane. 

Second.  It  shows  which  muscles  are  involved  in  the  pro- 
duction of  these  or  other  rotations,as  does  the  Knapp  model. 

Third.  It  shows  the  retinal  image,  as  does  the  Bonders 
model,  and,  besides,  the  relations  of  the  two  images  to  each 
other. 

Fourth.  It  is  simple  and  strong,  and  in  this  way  adapted 
to  the  rough  handling  always  given  by  students  to  such 
pieces  of  apparatus.  This  ophthalmotrope  and  the  others 


1 86  Action  of  a  Single  Muscle 

assist  the  teacher  in  conveying  those  fundamental  ideas 
concerning  the  movements  of  the  ocular  muscles  which  can 
not  be  given  in  any  other  way.  Also,  they  assist  the  more 
mature  student,  for  no  matter  how  large  may  be  the  experi- 
ence of  an  ophthalmic  surgeon,  he  is  certain  to  have  cases 
in  which  it  is  difficult  without  some  such  aid  to  form  a  clear 
mental  picture  of  just  what  rotations  the  eye  makes,  or 
what  muscles  are  involved  and  to  what  degree. 

§  3.  Action  of  a  Single  Muscle. — In  studying  the 
motions  of  the  globe,  it  is  logical  to  see  first  how  one  muscle 
acts  alone,  and  then  in  combination  with  other  muscles. 
Theoretically  this  is  very  simple,  for  each  muscle  makes 
traction  in  a  plane  which  is  determined  by  three  points — 
the  center  of  motion  of  the  globe,  and  the  insertion  and 
the  origin  of  that  muscle.  In  reality,  however,  the  problem 
is  not  so  simple.  The  center  of  motion,  it  is  true,  can  be 
determined  quite  exactly,  and  we  also  know  what  the  ana- 
tomical insertions  of  these  muscles  are.  But  we  must  distin- 
guish between  the  anatomical  and  \\\t physiological  insertion. 
The  latter  is  at  the  point  of  contact  of  the  muscle  with 
the  globe.  Now  this  point  and  the  arc  of  contact  of  any 
individual  muscle  evidently  change  with  each  motion  of 
the  eye.  Hence,  in  any  calculation  concerning  the  action  of  a 
muscle,  it  must  be  considered  as  if  inserted  at  the  point  of 
contact.  The  third  factor  in  this  question  is  also  a  little  con- 
fusing for  the  reason  that,  unfortunately  for  convenience  of 
calculation,  the  optic  foramen,  with  the  muscles  arising  around 
it,  does  not  lie  directly  behind  the  globe  and  on  a  level  with 
its  center,  but  toward  the  median  line  and  partly  above  but 
principally  below  the  horizontal  plane.  Thus  the  action  of 
the  internal  rectus,  for  example,  is  not  to  turn  the  cornea 
straight  inward,  as  we  usually  think,  but,  accurately  stated, 
to  turn  the  cornea  in  and  a  trifle  downward,  because  the 
center  of  the  origin  is  a  little  below  the  center  of  the  inser- 
tion. This  downward  tendency  must  be  compensated  for 
by  the  superior  rectus  or  inferior  oblique.  The  mathe- 
matical details  of  this  question  have  been  worked  out 
by  several  men,  by  Weiland  (B  439)  and  especially  by 
Schneller  (B  438). 


Opposing  Action  of  Muscles  187 

§  4.  Movement  in  Any  Direction  Is  a  Resultant  of 
Two  or  More  Actions. — Even  a  motion  apparently  so 
simple  as  the  turning  of  the  cornea  straight  inwards  is 
therefore,  in  a  strict  sense,  not  simple,  but  a  resultant  of 
traction  in  at  least  two  directions.  Indeed,  all  motions  are 
really  the  resultants  of  two  or  more  forces  acting  in  different 
directions,  as  when  an  oarsman  rows  across  a  stream  the 
course  which  he  traverses  is  a  diagonal,  the  resultant  of  the 
forces  exerted  by  himself  and  by  the  current. 

§  5.  The  Opposing  Action  of  Muscles. — We  have 
seen  that  the  movement  of  the  eye  in  any  direction  is  a 
complicated  action,  involving  the  contraction  of  at  least  two 
or  three  muscles.  But  that  is  only  a  portion  of  the  prob- 
lem, for  the  lines  of  force  which  draw  the  globe  in  one 
direction  must  be  accompanied  by  a  corresponding  relaxa- 
tion of  another  group  of  muscles  on  the  other  side  of  the 
eye.  This  second  group  we  are  accustomed  to  call  the 
opponents  of  the  first  group.  Moreover,  their  relaxation 
must  take  place  regularly  and  equally,  otherwise  there 
would  be  a  jerking  motion  of  the  globe,  or  a  twisting  of  the 
axis  with  a  consequent  distortion  of  vision.  We  can  photo- 
graph these  motions  of  the  globe,  as  we  shall  see  later,  and 
we  find  that  as  it  swings  from  one  side  to  the  other  it  does 
so  ordinarily  with  a  smooth  and  regular  motion,  though  in 
certain  paralytic  conditions  the  globe  wavers  and  halts  in  its 
course,  just  as  we  might  expect  when  the  contraction  of 
one  group  of  muscles  and  the  relaxation  of  the  opposing 
group  are  not  properly  co-ordinated. 

§  6.  Pre-eminent  and  Subsidiary  Muscular  Functions. 
— Some  writers  make  a  marked  distinction  between  the  so- 
called  pre-eminent  or  primary,  and  subsidiary  or  secondary 
muscular  functions,  and  one  might  suspect  that  this  was 
some  specially  characteristic  function  of  a  given  muscle  or 
group  of  muscles.  They  are,  however,  simply  terms  to 
express  the  greater  or  less  degree  of  force  which  is  exerted 
by  a  muscle  or  a  group  of  muscles,  as  compared  with 
another  muscle  or  group  of  muscles,  when  both  unite  in 
producing  a  certain  action.  To  return  again  to  our  oarsman, 
if  he  can  row  almost  straight  across  the  stream,  the  force 


1 88  Rotations  in  Various  Directions 

which  he  exerts  is  pre-eminent  or  primary,  and  that  of  the 
current  is  secondary.  In  other  words,  these  terms  express 
only  the  difference  in  degrees  of  force  exerted. 

§  7.  Muscles  which  Cause  Rotation  in  Certain  Di- 
rections.— From  what  has  gone  before,  it  is  easy  to  construct 
a  table  showing  which  muscles  are  called  into  action  in  order 
to  rotate  the  globe  in  any  given  direction.  Such  tabular 
arrangements  are  given  in  most  of  the  text-books,  and  are 
usually  about  as  follows : 

UPWARD,  Superior  rectus  and  inferior  ob- 

lique. 

D  WNWARD,  Inferior  rectus  and  superior  ob- 

lique. 

INWARD,  Internal  rectus  and  superior  and 

inferior  recti. 

OUTWARD,  External  rectus  and  superior  and 

inferior  oblique. 

UPWARD  AND  INWARD,  Superior  rectus,  internal  rectus, 

and  inferior  oblique. 

UPWARD  AND  OUTWARD,  Superior  rectus,  external  rectus, 

and  inferior  oblique. 

DOWNWARD  AND  INWARD,  Inferior  rectus,  internal  rectus, 

and  superior  oblique. 

DOWNWARD  AND  OUTWARD,  Inferior  rectus,  external  rectus, 

and  superior  oblique. 

The  recti  muscles  tend  to  draw  the  eye  into  the  orbit,  while 
the  oblique  muscles  tend  to  draw  it  out. 

For  physiological  purposes  now,  and  also  when  dealing 
with  the  diagnosis  of  the  paralyses  later,  it  is  well  to  group 
together  those  muscles  which  turn  each  eye  to  the  right  and 
those  which  turn  it  to  the  left  as  follows : 

DEXTRODUCTORS.  Right   external    rectus   with   the  su- 

perior and  inferior  oblique. 
Left  internal  rectus  with  the  superior 

and  inferior  recti. 
IWEVODUCTORS  Left  external  rectus  with  the  superior 

and  inferior  oWique. 
Right  internal  rectus  with  the  superior 
and  inferior  recti. 


Field  of  Fixation  189 

DEXTRAL  SUPERDUCTORS.    Right  superior  rectus  with  the  exter- 
nal rectus  and  inferior  oblique. 
Left  superior  rectus  with  the  internal 

rectus  and  inferior  oblique. 
LUEVAL  SUPERDUCTORS.        Left  superior  rectus  with  the  external 

rectus  and  inferior  oblique. 
Right  superior  rectus  with  the  internal 

rectus  and  inferior  oblique. 
DEXTRAL  SUBDUCTORS.        Right  inferior  rectus  with  the  external 

rectus  and  superior  oblique. 
Left  inferior  rectus  with   the  internal 

rectus  and  superior  oblique. 
LJEVAL  SUBDUCTORS.  Left  inferior  rectus  with  the  external 

rectus  and  superior  oblique. 
Right  inferior  rectus  with  the  internal 
rectus  and  superior  oblique. 

§  8.    Field  of  Fixation  and  Methods  of  Measuring  it. — 

All  the  points  which  an  eye  can  see  or  "  fix  ,"  while  the  head 
meanwhile  remains  immovable,  constitute  the  field  of  fix- 
ation or  the  motor  field.  When  we  reach  the  pathological 
aspects  of  our  subject,  we  shall  find  that  even  slight  variations 
in  the  limits  of  this  field  have  in  some  cases  an  important 
bearing  on  questions  of  diagnosis  and  treatment.  Nearly  all 
of  the  text-books  point  out  the  desirability  of  ascertaining 
the  extent  of  these  rotations,  if  only  to  recognize  cases  of 
suspected  paresis,  but  the  real  fact  is  that  such  measure- 
ments are  seldom  made.  Probably  this  is  because  the  meth- 
ods thus  far  employed  are  unsatisfactory.  We  can  measure 
the  extent  of  this  field  either  subjectively  or  objectively. 

(A)  The  subjective  method  consists  in  having  the  patient 
look  with  one  eye  at  an  object  when  that  is  moved  through 
a  given  arc,  in,  out,  up,  down,  or  in  any  oblique  direction. 
Most  of  the  records  given  in  the  text-books  were  obtained  in 
this  way  with  the  aid  of  the  perimeter.  But  it  is  often 
difficult  to  decide  whether  the  test  object  is  seen  directly,  or 
with  a  part  of  the  retina  which  is  more  or  less  eccentric. 
Even  one  who  is  accustomed  to  laboratory  methods  will 
readily  make  this  mistake.  In  order  to  obviate  that  difficulty, 
Wood  (B  456)  attached  on  the  inside  of  the  perimeter,  a 


The  Perimeter 


strip  of  paper  on  which  words  of  two  letters  are  printed. 
The  ability  to  read  these  words  when  the  arc  is  placed  in 
different  meridians  indicates  the  extent  of  the  rotation  of 
the  globe. 

(B)  The  objective  methods  of  measuring  the  field  of 
fixation  are,  in  general,  the  most  satisfactory,  although  for 
this,  suitable  apparatus  is  required.  Here  again  most  physi- 
ologists and  clinicians  have  depended  on  the  perimeter. 

The  Perimeter. — After  many  trials  with  various  forms  of 
this  instrument,  I  have  found  that  the  excursions  of  the  globe 
can  be  determined  most  satisfactorily  by  attention  to  the 
following  points. 

First,  to  lessen  the  sources  of  error,  the  instrument  as  a 
whole  should  be  of  considerable  size.  One  much  used  in 
this  country,  having  a  diameter  of  fifty-five  centimeters,  is 
none  too  large.  It  has  been  found  that  various  small 
changes  could  be  made  in  this  instrument,  as  shown  in 

Fig-  137- 

(a)  The  arc  should  be  graduated  in  degrees  on  both  the  in- 

and  outside  of  the  band  for  convenience  in  reading:.     At  the 


FIG.  137. — Perimeter  with  a  gun-sight  and  also  a  telescope  attachment  for 
measuring  the  field  of  fixation. 

zero  point  there  should  be  an  opening  five  or  ten  millime- 
ters in  diameter  through  which  it  is  easy  for  the  observer  to 
look  at  the  eye  which  is  being  measured.  In  addition,  there 


The  Perimeter  191 

are  two  slots  s  s  for  another  purpose  which  will  be  men- 
tioned later.  When  centering  the  globe  in  the  arc  of  the 
perimeter  we  have  the  subject  look  through  the  circular  open- 
ing at  the  zero  point,  and  if  the  eye  of  the  observer  is  on 
the  other  side  of  this  opening  it  is  easy  to  see  that  the  radius 
of  the  perimeter  at  that  point  passes  also  through  the 
observed  eye. 

But  the  eye,  while  still  in  the  line  of  that  radius  may  be  too 
near  the  perimeter  or 
too  far  from  it.  Ordina- 
rily we  then  move  the 
head  of  the  subject  a 
little  farther  forward  or 
backward,  as  the  case 
may  be,  and  try  to  sight 
across  the  arc  from  one 
of  its  extremities-  .to 
the  other,  at  the  point 

marked  00°.       In  doing       _ 

fa         FIG.    138. — Gun-sight   attachment   to    the 

this,  however,  the  ob-  perimeter  to  assist  in  bringing  the  center  of 
server  is  not  sure  that  the  eye  to  the  center  of  the  arc. 

his  own  eye  is  even  approximately  at  the  extremity  of  the 
arc. 

(d)  Gun-sight  Attachment  to  the  Perimeter. — In  order 
to  obtain  greater  accuracy  in  this  respect,  it  is  convenient 
to  have  a  carrier  or  band  which  slides  along  the  arc  and  to 
which  a  small  gun-sight  is  attached  (Fig.  138).  This  is  noth- 
ing more  than  a  strip  of  brass  about  five  centimeters  long 
and  four  millimeters  wide.  The  ends  are  bent  at  right  angles 
to  the  rest  of  the  strip,  one  is  filed  into  a  point,  and  the  other 
indented  as  a  notch.  The  central  portion  of  the  gun-sight 
is  fixed  to  the  top  of  the  carrier  with  a  hinge,  and  is  ad- 
justed carefully  at  right  angles  to  the  arc  so  that  wherever 
placed,  it  is  in  line  with  a  radius  of  the  perimeter  at  that 
point.  There  is  also  attached  to  the  inner  surface  of  the 
carrier  a  small  graduated  arc,  its  center  being  the  same  as 
the  hinge  on  which  the  gun-sight  tips.  Its  purpose  is  to 
mark  the  number  of  degrees  which  the  gun-sight  must  be 
depressed  in  order  to  point  at  a  certain  portion  of  the  globe. 


192  The  Perimeter 

While  this  small  arc  is  rather  convenient,  it  is  not  necessary. 

After  the  globe  has  been  brought  into  line  with  the  radius 
which  passes  through  the  central  point  of  the  perimeter,  the 
carrier  containing  the  gun-sight  is  slipped  on  the  arc  and 
the  observer  sights  across  this  at  the  eye  of  the  subject. 
It  is  still  impossible  to  know  with  absolute  exactness  that  a 
given  radius  passes  through  the  center  of  motion  of  the 
globe,  but  if  the  observed  eye  be  brought  to  a  position  near 
the  center  of  the  arc,  so  that  the  radius  marking  80  or  90 
degrees  on  the  perimeter  impinges  upon  the  observed  eye 
just  posterior  to  the  cornea,  as  indicated  by  the  gun-sight, 
and  if,  at  the  same  time,  the  globe  be  in  the  line  of  the 
radius  passing  through  the  zero  point,  as  just  described,  the 
center  of  motion  of  the  globe  then  corresponds  practically 
to  the  center  of  the  arc  of  the  perimeter. 

(c)  Telescope  Attachment  to  the  Perimeter. — While  the  gun- 
sight  enables  us  to  make  more  accurate  observations  than 
with  the  naked  eye,  still  more  constant  results  can  be 
obtained  in  measuring  the  excursions  of  the  eye  by  mount- 
ing a  small  telescope '  on  a  carrier  in  the  same  way  as 
the  gun-sight — that  is,  exactly  in  the  direction  of  a  radius  of 
the  perimeter.  Another  carrier,  weighted  with  a  lead  band 
must,  however,  then  be  placed  on  the  opposite  arm  of  the 
perimeter  to  act  as  a  counterpoise.  One  great  advantage 
of  the  telescope  over  the  gun-sight  is  that  when  the  eye 
turns  outward  to  a  point  where  it  reaches  nearly  the  limit  of 
fixation,  and  remains  there  but  for  an  instant  and  perhaps 
with  a  tremulous  motion,  we  can  observe  its  behavior 
through  the  telescope  better  than  when  simply  viewing  it 
with  the  naked  eye. 

(cT)  Electric-Light  Attachment. — The  objective  examina- 
tion can  also  be  made  more  accurately  by  attaching  a  small 
electric  light  to  the  carriers.  This  had  already  been  done 
in  measuring  the  field  of  vision.  When,  however,  such 

1  These  telescopes,  measuring  twelve  centimeters  long  and  having  a  diameter 
of  two  and  a  half  centimeters,  are  sold  by  several  opticians  at  two  or  three 
dollars.  It  is  necessary,  however,  to  place  in  front  a  spherical  glass  of  about 
twenty-two  diopters  in  order  to  shorten  the  focal  distance  sufficiently  to 
make  them  useful  on  the  arc  of  a  perimeter. 


Tropometers  193 

a  light  is  placed  just  below  the  gun-sight  or  the  small 
telescope,  then  as  the  patient's  eye  follows  the  light,  the 
observer,  looking  over  the  gun-sight  or  through  the  tele- 
scope, sees  the  bright  reflection  just  in  the  center  of  the 
pupil.  In  this  way,  still  greater  accuracy  is  possible  in 
deciding  how  far  the  globe  has  rotated.  The  battery  for 
the  lamp  is  a  small  dry  cell,  such  as  is  found  in  any  of  the 
electrical  supply  shops.  By  a  spring  switch,  the  lamp  is 
lighted  or  extinguished  as  desired,  the  whole  being  simple 
and  inexpensive.  Of  course  it  is  unnecessary  to  make  use 
of  both  the  gun-sight  and  the  telescope.  For  most  pur- 
poses the  former  is  sufficient,  although  it  is  not  entirely 
accurate. 

If  we  depend  exclusively  on  either  the  subjective  or  the 
objective  methods,  errors  are  apt  to  occur.  It  is  therefore 
desirable  to  employ  both,  and  with  a  little  practice  the 
measurement  can  be  made  quickly  and  easily.  But  in  spite 
of  every  precaution  in  the  construction  and  use  of  the  per- 
imeter,  it  is  still  an  imperfect  instrument  for  measuring 
the  excursions  of  the  globe.  As  the  nose  projects  in  the 
median  line  and  the  brow  above,  and  as  it  is  impossible  to 
sight  over  the  arc  from  below  upwards,  the  view  in  these 
directions  is  of  course  restricted.  These  difficulties  led  to 
the  construction  of  the  tropometer. 

The  Tropometer — Many  of  the  earlier  students,  appre- 
ciating the  great  clinical  importance  of  the  field  of  fixa- 
tion, have  made  efforts  to  measure  it  otherwise  than  with 
the  perimeter.  As  early  as  1876  Nicati  constructed  an 
instrument  which  he  called  a  tropometer  (B  451-453),  the 
object  being  to  determine  the  excursion  of  the  globe  by 
measuring  the  tangent  of  the  arc  through  which  it  rotated. 
Later  Stevens  (B  460)  described  more  complicated  in- 
strument constructed  on  the  same  general  principle  and 
called  by  the  same  name.  This  is  the  one  best  known  at 
present,  having  been  figured  by  Maddox,  De  Schweinitz, 
and  others,  but  ingenious  contrivances  for  accomplishing 
the  same  purpose  have  been  arranged  by  Eaton  (B  461) 
in  this  country  and  by  others  elsewhere. 

Stevens'  tropometer  (Fig.  1 39).  consists  essentially   of  a 


194 


Tropometers 


head-rest  of  rather  complicated  form,  and  a  telescope  with 
which  to  view  the  cornea.  Instead  of  looking  directly  at 
the  eye,  however,  this  telescope  is  placed  at  right  angles  to 
the  axis  of  vision  of  the  subject,  and  a  mirror  opposite  the 
telescope  at  an  angle  of  45  degrees  reflects  into  the  telescope 
the  image  of  the  cornea.  The  best  part  of  this  instrument 
is  a  tangent  scale  which  is  placed  in  the  eyepiece,  and 
which  shows  through  how  many  degrees  a  given  point  of 
the  cornea — or  really  how  far  the  edge  of  the  cornea — is 
rotated  in  any  direction. 

After  considerable    experience   with    Stevens'  tropome- 
ter,  I  have  found  that  it  is  entirely  unnecessary  to  turn 


FlG.  139. — Stevens'  tropometer. 

the  telescope  at  right  angles  to  the  axis  of  vision  of  the  eye 
examined.  After  a  time  I  removed  the  box  at  its  distal 
end,  and,  directing  the  tube  straight  at  the  patient's  eye,  a 
clearer  image  and  equally  accurate  measurements  were 
obtained.  Moreover,  it  was  found  that,  instead  of  multi- 
plying instruments,  it  was  easy  to  change  the  ophthalmic 
microscope,  (Fig.  117)  which  was  used  at  first  only  for  viewing 
the  pupil,  into  a  very  excellent  tropometer.  This  is  done 
by  placing  a  minus  spherical  in  front  of  the  object  glass  to 


Tropometers 


195 


lessen  the  magnification,  and   then  using  an   ocular  which 
contains  a  tangent  scale. 


eo 


40 


20 


60 


FIG.  140. — Tangent  scale  in  the  eyepiece  of  the  tropometer.  (Much  enlarged.) 

For  those  who  have  forgotten  the  exact  meaning  of  this 
term,  the  accompanying  diagram  (Fig.  140)  may  be  of  some 
assistance.  Thus,  if  different  radii  be  drawn  from  the 
point  C,  at  intervals  of  say  ten  degrees  each,  to  the  line  AB, 
which  is  tangent  to  the  arc  at  the  point  O,  then  the  points  at 
which  these  radii  intersect  the  tangent  AB  would  mark  off 
what  we  may  call  a  "  tangent  scale."  Instead  of  drawing 
this  one  line  horizontally  across  the  field  of  the  instrument  it 
is  better  for  our  purpose  to  have  a  number  of  parallel  lines 
whose  distances  from  each  other  are  the  same  as  the  distances 
at  which  the  different  radii  intersect  the  tangent  already 
referred  to.  Theoretically  it  would  be  quite  sufficient  to 
have  a  single  tangent  scale  stretching  across  the  field.  In 
practice,  however,  it  is  sometimes  more  convenient  to  have 
two  such  scales.  One  of  these,  numbered  from  right  to  left, 
measures  the  rotations  of  the  globe  in.  one  direction,  and 
another  scale,  numbered  from  left  to  right,  measures  the 
rotations  in  the  other  direction.  To  use  this  form  of  the 
tropometer,  it  is  only  necessary  to  turn  the  instrument  on  its 
vertical  axis  until  the  zero  point  of  the  scale  touches  the 


1,96  Extent  of  the  Field  of  Fixation 

edge  of  the  cornea.  Then  observe  through  how  many  di- 
visions of  the  scale  that  point  on  the  cornea  moves  when  the 
eye  is  rotated  in  a  given  direction,  and  we  have  at  once 
that  rotation  in  degrees.  If  it  is  desired  to  measure  the  excur- 
sion of  the  eye  up  or  down,  we  turn  the  eyepiece  so  that  the 
scale  is  vertical,  and  have  the  lid  of  the  eye  which  is  under 
examination  lifted  so  that  the  edge  of  the  cornea  is  distinctly 
visible.  The  rotations  of  the  globe  up  and  down  are  then 
observed  in  the  same  way  as  when  in  the  horizontal  plane. 

When  the  ophthalmic  microscope  is  thus  changed  into  a 
'tropometer,  the  subject,  sitting  in  front  of  the  instrument, 
ordinarily  steadies  the  head  by  resting  his  elbows  on  the 
table,  and  supporting  his  chin  on  the  palms  of  his  hands. 
That  is  usually  sufficient  for  clinical  purposes.  For  labora- 
tory work,  however,  or  when  special  exactness  is  desired,  it 
is  well  to  use  the  head-rest  already  described  (Fig.  1 17). 

§  9.  Extent  of  the  Field  of  Fixation. — The  limits  of 
this  field,  as  found  by  earlier  students  of  the  subject,  are 
shown  in  the  following  table  taken  mainly  from  Landolt 
and  Eperon. 

Schuurmann,  Volkmann,  Hering,  Kuster,  Schneller,  Landolt,  Duane. 
Abduction         42  38  43  43  4&~54  46  53 

Adduction         45  42  44  45  52-56  44  5* 

Superduction   34  35  20  33  44  43 

Subduction        57  50  62  44  50  63 

The  field  of  fixation  as  shown  by  Schuurmann  and  Landolt 
is  seen  in  Fig.  141  and  Fig.  142. 

The  difficulties  of  ascertaining  the  exact  limits  of  the 
field  of  fixation  are  shown  by  Duane.  He  gives  the  results 
of  the  several  measurements  made  on  the  same  individual, 
and  his  figures  prove  what  can  be  easily  verified,  that  it 
seldom  happens  that  any  two  tests  agree  exactly  in  all  their 
details. 

In  other  words,  we  know  quite  nearly  what  the  field  of 
fixation  is,  though  it  is  evidently  impossible  to  select  any 
special  number  of  degrees  which  shall  mark  the  limit  of 
motion  in  a  given  direction  in  any  individual  case. 

It  is  important  to  remember  that  the  limits  of  the  field  of 


Extent  of  the  Field  of  Fixation 


197 


34° 


M.   38 


E.     42° 


H.     38° 


-41°  M. 


-45°   E. 


-47"  H. 


(Nose) 


57C 


fixation  are  not  to  be  judged  entirely  by  the  number  of 
degrees   which    an    eye   can    turn    in  any    given    direction. 

Much  also  depends  upon  the 
manner  in  which  that  motion 
is  made.  In  some  individuals, 
when  the  group  of  muscles 
under  examination  is  strong,  the 
eye  will  turn  without  hesitation 
to  a  certain  point  on  the  arc  of 
the  perimeter  which  is  the  limit 
of  motion  in  that  direction, 
and  the  globe  can  be  held  in 
that  position  steadily  and  with- 
out effort  for  several  seconds. 
On  the  other  hand,  when  this 
group  of  muscles  is  weak  and 
the  eye  approaches  the  limit  of 

fixation,  it  does  so  in  a  halting  and  almost  tremulous  fashion. 
It  remains  at  that  point  only 
an  instant,  and  at  once  swings 
back  into  a  more  usual  posi- 
tion. The  beJiavior  under 
such  circumstances  is  most 
important  from  the  clinical 
standpoint  in  showing  not 
simply  the  limit  of  the  ex- 
cursions, but  also  the  ability 
to  reach  it.  This  fact  will  be 


FIG.  141. — Limits  of  field  of 
fixation  in  myopia,  emmetropia, 
and  hypermetropia  (Schuur- 
mann). 


47 


Nose 


FIG.   142. — Limits  of  field  of  fixa- 
tion  (Landolt). 


referred  to  more  than  once 
when  this  subject  is  studied 
from  the  clinical  aspect. 

§  10.  Of  what  Clinical  Value  is  a  Knowledge  of  the 
Field  of  Fixation  ? — After  devoting  so  much  time  to 
the  discussion  of  instruments  for  measuring  the  field  of 
fixation  and  to  the  results  obtained  by  them,  we  natu- 
rally ask — what  of  it?  In  order  to  answer  this,  let  us 
presuppose  for  a  moment  an  acquaintance  with  the  pa- 
thological aspects  of  our  study  and  recall  the  points  which 
we  desire  to  learn  in  any  case  of  heterotropia, — as,  for 


198        Clinical  Value  of  the  Field  of  Fixation 

example,    in    an    abnormal    convergence.        We    wish   to 
know: 

1st.     Does  any  deviation  of  one  eye  or  both  exist? 

2d.     Which   eye — if  either — is  especially  affected  ? 

3d.     Exactly  which  muscle  or  group  of  muscles  is  affected? 

4th.  Is  the  deviation  due  to  excessive  contraction  of  the 
adductors  (active  esotropia),  or  to  paresis  of  the  abductors 
(passive  esotropia),  or  is  it  due  to  both  causes? 

Some  practitioners,  it  is  true,  do  not  take  the  time  and 
trouble  to  ask  these  questions.  For  them  it  is  sufficient  to 
know  that  a  deviation  does  exist ;  they  recognize  only  the 
more  evident  forms,  and  usually  make  some  operation 
promptly  for  all  varieties.  Such  criminal  carelessness  re- 
quires no  comment. 

But  when  an  attempt  is  made  to  answer  any  of  these 
questions  too  much  dependence  is  often  placed  on  the 
tests  with  double  images,  such  as  will  be  considered  in  the  part 
relating  to  paralyses.  But  such  tests,  like  subjective  tests 
with  the  perimeter  or  with  any  similar  instrument,  have  two 
important  defects.  They  presuppose  sufficient  vision  in 
each  eye  to  recognize  the  test  objects,  and  also  sufficient  in- 
telligence in  the  subject  for  exact  replies.  Now  the  fact 
is,  that  many  of  our  cases  are  deficient  in  one  or  both  of 
these  qualifications — for  example,  most  children,  or  un- 
educated adults,  especially  those  of  dispensary  or  hospital 
practice,  and  others  unnecessary  to  specify  here.  Moreover, 
in  all  subjective  tests,  no  matter  how  good  the  vision  or  how 
intelligent  the  patient  may  be,  still  another  element,  the 
personal  equation,  must  be  taken  into  account.  Evidently, 
therefore,  if  we  would  obtain  more  than  the  most  superficial 
knowledge  of  these  important  deviations,  we  must  collect  all 
the  data  we  can  by  objective  measurements.  This  applies 
not  simply  to  the  limits  of  the  field  of  fixation,  but  to  the 
rapidity  with  which  the  globe  swings  from  side  to  side,  to 
the  lifting  power  of  the  adductors,  and  possibly  also  to  the 
muscle  sound — all  of  which  are  to  be  considered  presently. 

Having  thus  glanced  at  the  reasons  why  the  objective 
methods  of  examination  are  preferable  to  the  subjective, 
let  us  ask  more  exactly  in  what  \\ay  the  measurements  of 


Clinical  Value  of  the  Field  of  Fixation        199 

the  field  of  fixation  enable  us  to  answer  one  or  more  of  the 
four  questions  already  referred  to. 

1st.  As  to  determining  whether  or  not  a  deviation  does 
exist.  When  that  is  not  always  apparent  ( heterotropia ) 
but  usually  latent  (heterophoria),  the  contraction  or  exten- 
sion of  this  field  may  alone  indicate  the  deviation.  That 
may  not  be  shown  entirely  by  the  limits  of  the  field,  but 
quite  as  much  by  the  behavior  of  the  eye  as  it  approaches 
those  limits,  as  already  indicated.  In  this  connection  it  is 
only  possible  to  refer  briefly  to  this  diagnostic  point. 

2d.  Any  difference  between  the  limits  of  the  field  of 
fixation  in  one  eye  as  compared  with  the  limits  in  the  other 
eye,  undoubtedly  does  assist  very  materially  in  the  conclu- 
sions. Most  of  the  tests  with  double  images,  especially  those 
for  determining  the  static  position  of  the  globe,  do  not  show 
conclusively,  and  sometimes  not  at  all,  in  which  eye  the 
difficulty  lies.  On  the  other  hand,  any  one  accustomed 
to  make  measurements  of  the  field  of  fixation  of  each  eye» 
will  appreciate  that  in  a  considerable  percentage  of  cases 
heterotropia  is  accompanied  by  a  limitation  of  the  field 
in  one  eye  entirely,  or  at  least  in  one  as  compared  with  the 
other. 

3d.  Almost  as  a  corollary  from  the  last  statement,  it 
follows  that  the  limits  of  this  field  assist  in  locating 
the  group  of  muscles,  or  even  the  principal  muscle  affected. 

4th.  Finally,  in  cases  of  heterotropia  the  question  always 
arises,  does  the  eye  before  us  deviate  because  it  is  drawn 
out  of  place  by  excessive  traction  of  one  group  of  muscles, 
or  because  of  imperfect  innervation  of  the  opposing  group  ? 
This  question  is  so  difficult  to  decide  that  we  need  the  help  of 
all  the  methods  of  investigation  at  our  command.  And  in 
this,  measurements  of  the  field  of  fixation  apparently  do 
assist,  at  least  to  some  extent.  For,  in  a  given  case  of  eso- 
tropia,  if  the  eye  can  be  turned  outward  the  usual  amount, 
or  even  more,  then  we  are  safe  in  assuming  that  the  abduc- 
tors have  quite  a  sufficient  innervation,  and  probably  the 
esotropia  is  due  to  excessive  action  of  the  adductors.  It 
must  be  understood,  of  course,  that  such  evidence  of  itself 
would  be  unreliable,  but  when  that  is  corroborated  by  other 


200 


Lifting  Power  of  the  Adductors 


data,  the  conclusion  is  at  least  more  warrantable  than  with- 
out such  evidence. 

§  ii.  Lifting  Power  of  the  Adductors. — Having 
ascertained  by  what  muscles  the  eye  is  rotated  in  a  given  di- 
rection and  also  the  limits  of  that  rotation,  we  may  ask  next 
what  amount  of  force  they  actually  exert,  or  what  is  their 
lifting  power.  An  attempt  has  been  made  to  determine  this. 


FIG.  143. — Arrangement  for  measuring  the  lifting  power  of  the  adductors. 

The  method  is  shown  in  Fig.  143.  The  eye  being  under 
cocain,  a  speculum  is  introduced,  and  a  pair  of  small  forceps 
made  especially  for  this  purpose  is  fastened  to  the  con- 
junctiva over  the  insertion  of  the  external  rectus,  grasping 
firmly  also  the  tendon  of  that  muscle.  The  thread  which  is 
attached  to  the  forceps  is  made  to  run  obliquely  backward 
over  a  roller  and  is  connected  below  with  an  open  dish. 
Water  is  then  injected  into  the  dish  from  a  small  syringe  until 
the  eye  begins  to  move  outward.  By  slightly  decreasing  or 
increasing  the  amount  of  water  in  the  dish,  it  is  possible  to 
reach  with  considerable  exactness  a  point  where  the  cornea 
is  just  held  in  position.  Then,  weighing  the  dish  with  its  con- 
tents, we  have  quite  nearly  the  lifting  power  of  these  muscles. 
It  is  not  easy  at  first  to  decide  exactly  what  weight  can  thus  be 
sustained  by  the  adductors,  for  when  the  limit  is  nearly  reached, 
each  addition  to  the  weight  causes  the  globe  to  yield  sud- 


Tensile  Strength  of  the  Recti  201 

denly,  although  it  tends  at  once  to  resume  its  former  posi- 
tion. With  a  little  practice,  however,  one  can  determine  quite 
well  how  many  grams  can  thus  be  sustained.  If  greater  exact- 
ness is  desired,  it  can  be  obtained  by  viewing  the  edge  of  the 
cornea  through  the  ophthalmic  microscope  fitted  with  a  tan- 
gent scale.  This,  however,  is  too  cumbersome  for  clinical 
purposes. 

The  results  of  these  measurements,  briefly  stated,  are  as 
follows.  When  a  person  looks  at  an  object  directly  in  front, 
the  force  exerted,  expressed  in  weight,  ranges  apparently  from 
ten  to  eighteen  grams,  with  an  average  of  about  fourteen 
grams.  It  is  quite  probable  that  these  figures  will  need  revi- 
sion when  measurements  have  been  made  of  a  larger  number 
of  normal  eyes.  For  although  these  are  evidently  the  most 
desirable  for  such  experiments,  it  is  difficult  to  find  subjects 
who  will  submit  to  the  inconvenience.  The  question  is  an 
interesting  one,  and  apparently  the  results  are  of  some  im- 
portance clinically.  In  a  case  of  esotropia,  for  example, 
the  lifting  power  of  the  adductors  constitutes  at  least  cor- 
roborative testimony  in  deciding  the  question  whether  the 
eye  tends  to  turn  in  from  excessive  action  of  the  adductors 
or  from  insufficient  action  of  the  abductors.  On  this  ques- 
tion depend  to  a  certain  extent  the  diagnosis  and  the  form 
of  treatment — certainly  if  that  be  of  an  operative  nature.  It 
helps  us  to  decide  whether  to  make  advancement  of  the  ex- 
ternus  or  tenotomy  of  the  internus. 

§  12.  Tensile  Strength  of  the  Recti. — The  power  of  the 
adductors  to  lift  a  given  number  of  grams  should  not  be 
confused  with  what  may  be  called  the  tensile  strength — that 
is,  the  weight  which  a  muscle  will  sustain  without  break- 
ing. Dianoux  (  B  464 )  gave  this,  in  dogs,  as  about  five  kilos. 
As  that  seemed  rather  large,  a  few  simple  tests  were  made  with 
the  recti  and  with  other  muscle  tissue.  These  were  interest- 
ing and  can  be  easily  repeated.  If  a  piece  of  beef  be  cut 
parallel  with  its  fibers,  trimmed  so  that  it  is  the  size  of  an 
internal  rectus,  and  suspended  with  a  weight  attached  to  the 
lower  end,  it  will  break  promptly  when  the  weight  reaches 
about  one  and  a  half  kilos.  Therefore  it  was  sup- 
posed that  the  recti  might  also  break  with  an  equally  light 


2O2  Rapidity  of  the  Lateral  Motion 

weight.  But  that  is  not  the  case,  perhaps  because  of  the 
abundance  of  fibrous  tissue  of  which  they  are  composed. 
For  example,  one  end  of  an  internal  rectus  was  attached  to 
a  rod,  while  the  lower  end,  having  a  clamp  attached  to  it,  held 
a  small  pan,  into  which  weights  could  be  placed.  It  was 
found  that  the  muscle  would  suspend  easily  a  weight  of 
from  two  to  two  and  a  quarter  kilos  before  breaking. 

In  this  experiment,  if  the  muscle  is  tied,  the  cords, 
which  hold  it  above  and  below,  are  apt  to  slip  from  the 
ends  before  the  fibers  break,  and  only  with  special  care 
can  this  be  avoided.1  It  will  be  noticed  that  the  sustaining 
power  of  a  rectus  muscle,  determined  in  this  way,  although 
several  times  greater  than  that  of  ordinary  muscle  fiber, 
is  still  decidedly  less  than  the  five  kilos  which  are  given 
by  Dianoux  as  the  tensile  strength  of  the  recti.  This  differ- 
ence may  be  due  to  the  freshness  of  the  material  used  or  to 
the  method  of  conducting  the  experiment.  The  fact  is, 
however,  that,  in  certain  persons  at  least,  the  tensile  power  of 
the  recti  is  by  no  means  as  great  as  has  been  generally 
supposed.  The  clinical  bearing  of  this  will  be  appreciated 
later  in  connection  with  one  or  two  operations  upon  those 
muscles.  Thus  we  shall  see  that  in  the  so-called  Panas 
operation  fortenotomy,  a  hook  is  passed  under  the  insertion, 
and  the  globe  is  rotated  far  toward  the  opposite  side.  In 
this  forcible  stretching  of  the  muscles  there  is  evidently  real 
danger  of  their  rupture. 

§  13.  How  can  the  Rapidity  of  the  Lateral  Motion 
of  the  Eye  be  Measured  ? — We  are  indebted  to  Volkmann 
(B  466,  p.  275)  for  the  first  attempt  to  measure  the  rapid- 
ity with  which  the  eye  moves  from  side  to  side.  He  directed 
the  individual  to  look  quickly  from  right  to  left  and  took  the 
average  time  required  for  a  single  movement.  Helmholtz's 
plan  (B  257)  was  to  have  the  person  count  the  number  of 
electric  flashes  which  could  be  perceived  while  the  eye  was 
passing  from  one  point  to  the  other.  But  these  methods 
gave  varying  and  rather  unsatisfactory  results. 

1  Acknowledgment  should  be  made  to  Mr.  H.  H.  Buckman,  Jr.,  for  his 
assistance  in  making  these  measurements  of  the  tensile  strength. 


Rapidity  of  the  Lateral  Motion 


203 


More  recently,  Dodge  (B  468)  measured  the  rate  of  move- 
ment by  the  aid  of  photography.  This  was  a  step  in  advance. 
His  method  was  to  throw  a  beam  of  light  upon  the  cornea 
and  have  it  reflected  into  a  camera.  The  plate-holder  of  the 
latter  contained  a  narrow  horizontal  slot  behind  which  a 
glass  negative  was  made  to  fall.  This  plate-holder,  which 
Dodge  mentions  as  an  important  part  of  his  apparatus,  was  a 


FIG.  144. — Arrangement  for  photographing  by  daylight  the  time  required 
for  an  eye  to  swing  through  a  given  arc. 

complicated  affair,  containing  a  small  tank  of  oil  through 
which  a  piston  passed  to  regulate  the  time  of  the  fall  of  the 
plate.  It  seemed  therefore  that  if  these  measurements  could 
be  simplified  and  adapted  to  hospital  or  possibly  to  office 
work,  by  showing  when  an  eye  moved  most  rapidly  and 
therefore  most  easily  in  one  direction  or  the  other,  they 
mishit  thus  give  valuable  hints,  at  least,  as  to  the  condition 

o  o 

of  the  muscles. 

Accordingly,  the  following  arrangement  for  photograph- 
ing with  daylight  the  rapidity  of  the  lateral  motion  was 
devised  (Fig.  144). 

First.  The  head-rest.  This  has  already  been  described. 
It  was  screwed  firmly  upon  the  end  of  the  table  opposite  to 
the  source  of  light,  and  by  changes  in  the  adjustment  the 
position  of  the  head  could  be  altered  as  desired. 


204  Rapidity  of  the  Lateral  Motion 

Second.  The  camera.  For  the  earlier  experiments1  the 
camera  used  was  the  Eastman  plate  camera  No.  3.  The 
only  alteration  in"  this  was  the  cutting  of  a  horizontal  slot 
about  two  millimeters  wide  across  the  center  of  the  thin  hard- 
rubber  slide  in  front  of  the  film.  The  latter  came  from  the 
manufacturer  on  one  roll,  arranged  to  unwind  upon  another 
roll  by  means  of  a  small  crank  moved  by  hand.  In  spite  of 
this  crude  method,  with  a  little  experience  that  camera  gave 
excellent  results.  It  can  be  improvised  without  difficulty. 
In  later  measurements,  trials  were  made  of  the  Century 
camera,  attaching  to  it  the  same  roll-holder  used  with  the 
first  instrument.  The  roll  was  then  turned  by  a  small  elec- 
tric motor,  also  attached  to  the  back  and  connected  to  the 
roll-holder  by  a  simple  gearing.  The  power  consisted  of 
three  dry  cells  of  the  United  States  Company,  No.  3.  The 
substitution  of  a  motor  for  hand  power  was  a  gain  in  one 
respect,  the  resulting  picture  being  much  more  uniform.  It 
had  the  disadvantage,  however,  of  requiring  still  another 
piece  of  machinery,  and  the  inevitable  jarring  produced  a 
wavy  effect  in  the  picture. 

Third.  Time  record.  To  obtain  this  a  small  mirror 
was  at  first  placed  outside  the  window.  That  reflected  a 
beam  of  light  upon  the  tip  of  a  tuning-fork,  which  was  con- 
structed to  make  fifty  vibrations  in  a  second.  By  properly 
adjusting  the  position  of  the  tuning-fork,  the  ray  of  light  was 
reflected  into  the  camera  in  such  a  manner  that  it  fell  upon  the 
horizontal  slot  in  the  rubber  slide,  which  was  in  front  of  the 
film.  In  the  later  experiments  electric  illumination  was  sub- 
stituted for  daylight.  One  of  the  so-called  Adams-Stagnal 
arc  lamps  was  placed  in  circuit  on  the  ordinary  alternating 
iO4-volt  current,  and  had  the  great  advantage  over  sunlight 
that  it  was  available  at  any  time.  This  arrangement  is  shown 
in  Fig.  145. 

Fourth.  Arc  of  rotation.  In  order  to  determine  the 
distance  in  degrees  traversed  by  the  eye,  an  arc  was  drawn 
on  the  table  a  certain  distance  in  front  of  the  eye,  and 

1  In  referring  to  these  experiments,  acknowledgment  should  be  made  of  the 
faithful  assistance  rendered  by  Mr.  Lionel  Duschak,  later  of  the  Depart- 
ment of  Chemistry  at  the  University  of  Michigan. 


Rapidity  of  the  Lateral  Motion 


205 


at  points  in  this  arc,  as  desired,  two  knitting-needles  were 
placed,  to  each  of  which  was  attached  a  piece  of  paper  or  other 
object  suitable  for  fixation.  When  the  person  looked  from 
one  of  the  knitting-needles  to  the  other,  the  eye  would  of 
course  swing  through  an  arc  of  known  length. 


FIG.  145. — Arrangement  for  photographing  by  electric  light  the  time  required 
for  an  eye  to  swing  through  a  given  arc. 

Finally,  it  was  found  convenient  to  have  the  whole  arrange- 
ment on  a  table  about  five  feet  high,  to  obviate  the  necessity 
of  constant  stooping. 

The  manner  of  making  the  exposures,  while  simple  in 
principle,  requires  attention  to  several  details.  The  subject's 
face  is  so  lighted  that  the  reflection  from  the  cornea  enters 
the  camera,  and  is  focused  about  the  center  of  the  slot  cover- 
ing the  film.  When  ready  for  an  exposure,  the  person  under 
examination  is  told  to  look  as  rapidly  as  possible  from  one  of 
the  knitting-needles  to  the  other,  the  tuning-fork  is  plucked 
and  the  roller  turned,  either  by  hand,  or  by  opening  the  cur- 
rent which  turns  the  rolls.  The  resulting  photographs  are 
easily  understood  (Fig.  146). 

When  the  eye  and  the  film  are  both  at  rest,  the  ray  of  light, 
reflected  from  the  cornea  through  the  slit,  is  focused  on  the 
film  as  a  bright  spot.  When  the  film  is  stationary  and  the 
eye  turns  from  side  to  side,  the  bright  spot,  moving  along 
the  horizontal  slit,  describes  a  horizontal  line  on  the  film. 
When  the  eye  is  stationary  and  the  film  is  made  to  move 
vertically  on  the  roller,  the  point  of  light  describes  a 


206 


Rapidity  of  the  Lateral  Motion 


FIG.  146. — Photographic  record  of  the  time  required  for  an  eye  to  swing 
through  a  given  arc.  The  line  on  the  left,  which  is  oblique  at  intervals,  is 
caused  by  the  reflection  from  a  point  on  the  cornea.  The  toothed  line  on  the 
right  is  the  reflection  from  a  tuning-fork,  whose  rate  of  vibration  is  known. 
The  number  of  vibrations  between  the  beginning  and  the  end  of  any  oblique 
portion  of  the  broken  line,  shows  the  time  required  for  the  eye  to  swing  from 
side  to  side. 

It  will  be  noticed  that  the  film  moved  more  rapidly  at  one  time  than  at 
another,  this  being  due  to  the  fact  that  it  was  unrolled  by  hand. 


Rapidity  of  the  Lateral  Motion  207 

vertical  line.  When,  however,  the  eye,  with  the  reflection 
from  it,  moves  horizontally,  and  at  the  same  time  the  film 
moves  vertically,  then  the  spot  of  light  describes  an  ob- 
lique line.  The  length  of  this  oblique  line  is  therefore  the 
measure  of  the  time  required  for  the  eye  to  swing  from 
one  side  to  the  other. 

When  the  results  obtained  in  the  manner  indicated  are 
compared  with  those  reported  by  Dodge,  they  are  very 
nearly  the  same.  The  differences  are  probably  accounted 
for  by  the  fact  that  his  measurements  were  based  on  the 
supposition  that  the  movement  of  the  cornea  was  the  same 
as  that  of  the  spot  of  light  reflected  from  it.  But  allowing 
for  this,  the  results  are  quite  as  constant  as  could  be  expected, 
and  show  that,  even  with  the  small  camera  containing  a 
film  moved  by  hand,  it  is  possible  to  measure  the  rate  and 
character  of  the  lateral  movements  of  the  eye  with  a  con- 
siderable degree  of  accuracy.  Photographs  made  in  this  way 
have  also  been  called  photograms  or  kinetograms.  When  at- 
tempting to  read  their  meaning,  a  superficial  glance  is  often 
deceptive.  As  the  simplest  and  best  method,  practically, 
to  move  the  film  on  the  roller  is  by  means  of  a  hand  crank, 
and  as  this  movement  of  the  hand  naturally  varies  in  speed 
from  one  instant  to  the  other,  the  rate  of  motion  of  even  a 
normal  eye  moving  with  perfect  regularity  may  appear  quite 
irregular  when  that  motion  is  depicted  on  a  film  which 
moves  irregularly.  This  is  indicated,  of  course,  by  the  vibra- 
tions of  the  tuning-fork  being  spread  out  in  some  places  and 
crowded  together  in  others. 

As  to  the  result  of  these  measurements,  it  must  be  ad- 
mitted that  they  are  not  constant  for  small  arcs —  from  five 
to  fifteen  degrees —  and  although  these  are  given  by 
Dodge  as  being  thirty  thousandths  of  a  second  for  a  swing 
through  an  arc  of  about  five  degrees,  and  about  forty 
thousandths  of  a  second  through  an  arc  of  ten  degrees,  my 
own  results  with  such  small  distances  are  too  irregular  to 
warrant  any  statements  on  that  point.  When,  however,  we 
measure  the  swing  through  an  arc  of  twenty  to  thirty 
degrees,  the  results  are  more  constant.  It  may  be  stated  in 
general  that  the  eye  requires  fifty  to  sixty  thousandths  of  a 


2o8  Rapidity  of  the  Lateral  Motion 

second  to  swing  through  an  arc  of  twenty  degrees,  about 
seventy-five  thousandths  to  swing  through  an  arc  of  thirty 
degrees,  and  about  one  hundred  thousandths,  or  one  tenth  of 
a  second,  to  swing  through  an  arc  of  forty  degrees. 

A  word  should  be  added  concerning  the  individual  charac- 
ter of  these  photograms  or  kinetograms.  The  figures  ob- 
tained from  different  tests  indicate  that  the  time  required 
for  the  eye  to  swing  through  a  given  arc  differs  somewhat  in 
different  persons.  It  is  quite  noticeable,  though,  that  a  cer- 
tain form  of  photograms  is  repeated  by  the  same  individual, 
indicating  that  the  eye  has  apparently  a  lateral  swing  which 
is,  to  a  certain  extent,  characteristic.  Its  behavior  during  the 
period  of  rest  between  its  swings  is  also  apt  to  be  characteristic. 
In  some  persons  it  remains  entirely  at  rest  at  the  end  of  the  arc 
of  rotation,  while  again,  especially  in  cases  of  paresis,  it  re- 
mains at  the  halting  point  trembling,  as  it  were,  in  its  place,  or 
it  may  begin  almost  at  once  to  move  back  to  the  other  end  of 
the  arc. 

What  is  the  clinical  value  of  a  knowledge  of  the  rapidity 
of  the  lateral  motion?  So  little  is  known  of  this  subject  that 
any  statements  as  to  its  value  must  be  made  with  caution. 
Such  facts  as  we  have,  however,  constitute  corroborative  evi- 
dence of  still  a  different  kind  as  to  whether  the  action  of  a 
given  set  of  muscles  is  or  is  not  entirely  normal.  Thus, 
when  a  question  arises  whether  an  abnormal  convergence  is 
due  to  a  contraction  of  the  adductors,  or  to  an  impaired  in- 
nervation  of  the  abductors,  if  the  eye  swings  toward  the 
median  line  much  more  rapidly  than  in  the  normal  condi- 
tion, it  is  probable  that  the  adductors  are  abnormally  strong. 
On  the  other  hand,  if  the  swing  inward  is  at  what  may  be 
called  a  normal  rate,  or  certainly  if  that  movement  inward 
is  less  rapid  than  normal,  we  may  incline  to  the  opinion 
that  the  difficulty  is  due  rather  to  a  relaxation  of  the  abduc- 
tors. Of  course  it  would  be  impossible  to  base  a  diagnosis 
on  such  evidence  alone,  but  these  data,  with  those  obtained 
by  other  methods  of  measurements,  may  furnish  the  basis  for 
a  valid  conclusion. 

§  14.  Movement  of  the  Eye  while  Reading. — 
Measurements  made  by  other  methods  have  shown  long  ago 


Movement  while  Reading  209 

that  as  the  eye  passes  along  a  line  from  left  to  right, 
in  the  act  of  reading,  it  stops  usually  four  or  five  times  for 
perhaps  five  or  ten  thousandths  of  a  second.  This  is  ap- 
parently in  order  that  the  brain  may  receive  the  impression 
made  upon  the  retina.  At  the  end  of  the  line,  the  eye  rests 
for  a  varying  length  of  time,  then  swings  back  to  the  left  side 
of  the  page  and  begins  the  same  journey  again.  It  is  not 
difficult  to  measure,  with  the  apparatus  referred  to,  the  rapid- 
ity with  which  an  eye  thus  moves.  Such  a  reading  record  or 
photogram  is  seen  in  Figure  147.  These  records  are  of  inter- 
est in  showing  the  really  complicated  nature  of  an  act  ap- 
parently so  simple  as  that  of  reading.  The  more  closely  this 
act  is  studied,  especially  by  these  photograms,  the  more 
readily  can  we  understand  how  it  may  become  difficult  or 


FIG.  147. — Drawing  from  a  photograph  of  an  eye  in  the  act  of  reading.  The 
zig-zag  line  on  the  right  shows  the  vibrations  of  a  tuning-fork. 

painful  when  there  exists  any  defect  in   either  accommoda- 
tion or  convergence. 

Dodge  (B  449)  says  that  when  the  eyes  turn  from  side  to 
side,  in  looking  at  distant  objects,  especially  when  such 
objects  are  moving,  occasional  halts  are  also  made  by  the 
eyes  as  in  the  act  of  reading.  Indeed,  there  seem  to  be 
five  and  possibly  more  kinds  of  lateral  movements  which 
can  be  distinguished  by  these  photographic  measurements. 


2io  The  Act  of  Winking 

§  15.  Measurement  of  the  Act  of  Winking. — As  the 
orbicularis  palpebrarum  is,  in  a  certain  way,  intimately  con- 
nected with  the  ocular  muscles,  we  should  notice  in  passing 
those  rapid  contractions  of  its  central  fibers  which  constitute 
the  act  of  winking.  The  method  of  measuring  by  photog- 
raphy the  time  occupied  in  the  act  of  winking  is  similar  in 
every  way  to  that  by  which  we  measure  the  length  of  the  time 
required  for  the  eye  to  swing  from  one  side  to  the  other. 
The  arrangement  of  the  light,  the  position  of  the  eye 
and  the  camera,  are  all  practically  the  same.  The  ray 
of  light  which  is  reflected  from  the  cornea  passes  through 
the  camera  and  through  the  small  horizontal  slit  in  the  slide' 


FIG.  148. — Corneal  reflection  broken  by  a  wink  of  the  subject. 

before  the  film.  The  film  is  then  moved  vertically  by  turn- 
ing the  crank.  The  ray  of  light  thus  reflected  on  the  cornea 
of  course  describes  a  vertical  line.  But  if  the  lid  is  closed, 
as  in  the  act  of  winking,  then,  the  cornea  being  covered,  the 
line  showing  the  reflex  from  it  is  of  course  also  interrupted. 
The  length  of  time  which  that  interruption  occupies  is  shown 
at  once  by  the  vibrations  of  a  tuning-fork  whose  rate  is 
known,  and  which  also  reflects  a  ray  of  light  through  the 
same  slit  upon  the  side  of  the  same  film.  This  is  seen  in 
Fig.  148.  These  photograms  show  that  the  act  of  winking 
requires,  from  first  to  last,  in  the  average  individual,  about 
one  half  of  a  second.  It  may  be  divided  into  three  portions. 
First,  the  time  occupied  in  the  closure  of  the  lids.  This  is  a 
sudden  movement,  and  the  picture  shows  that  the  line 
is  suddenly  broken  off  at  this  point.  That  part  of  the  act 


True  Torsion  with  One  Eye  2 1 1 

requires  from  about  one  tenth  to  one  twentieth  of  a  second. 

Second,  the  time  during  which  the  lid  remains  closed. 
This  also  varies,  in  different  individuals,  but  is  usually 
about  two  to  three  tenths  of  a  second. 

Third,  the  time  occupied  in  raising  the  lid.  This  part  of 
the  act  is  by  no  means  as  rapid  as  the  first ;  that  is,  the  lid 
is  raised  much  more  slowly  than  it  is  closed,  occupying  from 
one  to  two  tenths  of  a  second,  or  even  longer. 

It  may  be  asked  of  what  use  is  the  measurement  of 
the  act  of  winking?  When  it  is  decidedly  slower  in  one  eye 
than  in  the  other,  tha*t  fact  may  be  of  real  clinical  value, 
as  showing  that  the  corresponding  branch  of  the  third  nerve 
is  partly  paralyzed.  It  may  be  the  first  indication  of  a 
nuclear  paralysis.  That  symptom  alone,  or  with  others,  may 
point  to  the  location  of  an  existing  brain  lesion. 

§  16.  A  Wheel  Motion  (True  Torsion)  Possible 
with  One  Eye. — Thus  far  we  have  been  considering  the 
rotation  of  one  eye  about  some  axis  which  is  perpendicular 
to  the  antero-posterior  axis.  It  is  possible,  however,  for  a 
rotation  to  be  made  about  this  axis,  as  a  wheel  turns  on 
its  axle.  There  has  been  much  discussion  as  to  whether 
such  a  rotation  of  the  globe  is  possible  with  one  eye,  but 
the  point  was  settled  by  Javal  (B  264,  p.  298),  who  showed 
beyond  doubt  that  when,  in  viewing  a  distant  object,  he 
tipped  the  head  toward  one  side,  a  cylindrical  glass  no  longer 
gave  the  proper  correction,  as  the  upper  end  of  the  vertical 
axis  of  the  eye  did  not  tip  outward  as  far  as  did  the  glasses. 

Very  recently  this  subject  has  been  carefully  studied  again 
from  other  standpoints,  and  although  new  light  has  been 
thrown  upon  the  various  factors  which  modify  this  form  of 
torsion  of  one  eye,  the  underlying  facts  remain  the  same. 
While  it  is  interesting  to  know  that  it  is  possible  for  one  eye 
alone  thus  to  make  a  true  wheel  motion  about  the  antero- 
posterior  axis,  that  form  of  monocular  torsion  is  so  unusual 
as  to  be  of  no  practical  importance,  and  may  therefore  be 
left  with  this  brief  mention. 

§  17.  Sound  Produced  by  the  Eye  Muscles. — Maddox 
begins  his  book  on  the  ocular  muscles  by  remarking  on  their 
silence,  and  in  doing  so  reflects  the  popular  impression.  But 


212         Sound  Produced  by  the  Eye  Muscles 

more  than  a  quarter  of  a  century  ago  Hering  (B  480)  called 
attention  to  the  fact  that  their  motion,  especially  in  con- 
vergence, produces  a  rustling  sound,  which  could  be  heard 
when  listened  to  properly.  Hering  says :  "  The  change  in 
the  character  of  the  eye  sound  in  convergence  is  so  distinct 
as  to  be  recognized  at  once.  I  have  asked  several  observ- 
ers who  were  especially  proficient  in  auscultation  ...  to 
listen  while  I  made  the  experiment,  and  they  have  at  once 
been  able  to  decide  entirely  by  means  of  the  change  of  the 
sound  whether  or  not  my  eye  was  looking  in  the  distance, 
or  was  converged  for  a  near  point." 

That  observation  has  lain  buried  in  the  literature  all  these 
years,  and  yet  possibly  it  may  have  a  little  practical 
value.  Some  of  these  sounds  are  not  difficult  to  perceive, 
especially  with  a  proper  stethoscope  and  with  patience 
in  accustoming  the  ear  to  them,  but  after  a  considerable 
number  of  trials  I  must  confess  that  I  am  not  able  to  recog- 
nize them  with  the  confidence  expressed  by  Hering  and  his 
medical  friends. 

Thus  if  the  lids  be  closed  forcibly,  most  persons  can  recog- 
nize subjectively  a  faint 'rustling  sound.  It  can  be  distin- 
guished objectively  by  placing  on  the  eye  or  the  edge  of  the 
orbit  a  small  cone  of  hard  rubber — an  ear  speculum,  for  ex- 
ample— and  connecting  this  by  means  of  a  rubber  tube  with 
the  ear  of  the  listener,  though  I  have  found  that  the  muscle 
sound  could  be  recognized  more  easily  by  listening  to  it 
with  a  modified  form  of  the  double  stethoscope  (Fig.  149). 


FIG.  149. — Stethoscope  for  the  recognition  of  muscle  sound. 

This  art  must  be  learned,  just  as  with  the  use  of  the  stetho- 
scope for  detecting  delicate  variations  in  the  sounds  of  the 


Sound  Produced  by  the  Eye  Muscles        213 

lungs  or  heart.  It  is  true  that  if  the  stethoscope  be  placed  else- 
where, as  over  the  thick  part  of  the  corrugator  supercilii,  and 
that  muscle  be  strongly  contracted,  it  is  also  possible  to  per- 
ceive a  rustle,  apparently  different  from  the  rustle  heard 
over  the  orbit.  In  the  present  state  of  our  knowledge  it  can 
only  be  asserted  that  the  motions  of  the  ocular  muscles  can 
be  heard,  and  that  differences  in  their  sound  can  be  distin- 
guished, but  we  have  yet  much  to  learn  concerning  the 
relation  of  these  sounds  to  imperfect  muscular  action,  or  if 
they  have  any  clinical  value. 


CHAPTER    IV. 
BOTH   EYES   AT   REST. 

DIVISION   I. 
General  Considerations. 

§  i.  Both  Eyes  at  Rest.— All  that  has  been  said  thus 
far  pertains  or  may  pertain  to  a  single  eye  at  rest  or  in  motion. 
We  now  approach  another  set  of  physiological  phenomena 
in  which  the  relation  of  one  eye  to  the  other  must  be  taken 
into  account.  It  is  necessary  at  the  outset  to  recall  a  funda- 
mental principle  which  controls  all  associated  action  of  the 
two  eyes.  In  popular  language  this  is  called  "the  desire  for 
single  vision."  It  has  been  said,  more  tersely  than  exactly, 
that  "  Nature  abhors  double  vision  as  she  abhors  a  vacuum," 
or,  transposing  a  phrase  of  physics  into  terms  of  physiology, 
we  may  say  that  the  associated  motion  of  two  eyes  requires 
first  of  all  that  the  image  of  an  object  looked  at  shall  fall  on 
parts  of  the  two  retinas  which  correspond  to  each  other. 

The  main  facts  relating  to  corresponding  or  identical 
points  were  first  elaborated  by  Johannes  Mueller  and  were 
known  for  many  years  in  ophthalmic  literature  as  Mueller's 
rule.  This,  briefly  stated,  is  that  if  each  retina  were  divided 
into  quadrants  by  a  horizontal  and  a  vertical  meridian,  each 
of  which  passed  through  the  fovea,  and  if  we  were  to  imagine 
each  retina  to  represent  a  terrestrial  globe,  and  the  fovea  as 
a  point  of  the  equator,  then  identical  points  on  those  retinas 
might  be  described  as  having  the  same  latitude  and  longi- 
tude. This  is  a  simple  illustration  of  an  important  principle 
which  must  underlie  our  study  of  the  eyes  together,  whether 
at  rest  or  in  motion  (B  484-485). 

Mueller's  rule  is  sufficiently  exact  to  give  a  general  idea 
of  what  is  meant  by  corresponding  points  of  the  retina, 

214 


The  Interocular  Base  Line 


215 


although  that  rule  is  not  absolutely  true.  Our  first  ob- 
ject is  to  learn  the  position  which  each  eye  assumes  when 
it  is  in  a  state  of  so-called  "  rest,"  or  in  its  "  static  condi- 
tion." This  question,  which  we  now  approach  from  the 
physiological  standpoint,  is  of  so  much  importance  clinically 
that  it  can  be  separated  with  advantage  into  three  divisions. 
The  first,  includes  this  review  of  facts  which  relate  to  the 
subject  in  general  without  reference  to  any  special  pair  of 
axes.  In  the  second  division  we  will  consider  the  tests  by 
which  we  can  determine  the  position  of  the  visual  axes  when 
the  eyes  are  at  rest ;  and  in  the  third,  those  by  which  we  de- 
termine the  position  of  the  vertical  and  of  the  horizontal 
axes. 

§  2.  On  the  Measurement  of  the  Interocular  Base 
Line,  or  the  Distance  between  the  Centers  of  the  Eyes. 
— We  shall  have  occasion  to  use  this  distance  in  connection 
with  the  measurement  of  relative  accommodation  and  for 
other  purposes.  Every  ophthalmologist,  for  example,  appre- 
ciates the  necessity  of  estimating  this  distance,  at  least  rough- 


r«^i^f*i^v^Bw 

5  6  f       i       ? 

iiiiiiiiiiiiiiiiiiiiiiiii"iiiiiiii 


FIG.  150. — Gauge  for  estimating  roughly  the  distance  between  the  centers  of 
the  pupils. 

ly,  in  order  to  prescribe  properly  fitting  glasses,  particularly 
when  they  are  quite  strong.  The  base  line  may  be  determined 
either  subjectively  or  objectively.  The  former  method  was 
adopted  by  Smee  (6481),  who  constructed  a  double-barrel 
arrangement,  the  distance  between  the  barrels  being  adjust- 
able and  their  centers  corresponding  with  the  visual  axes. 


2l6 


The  Interocular  Base  Line 


All  Subjective  methods,  however,  are  specially  liable 
to  error  and  therefore  have  never  come  into  very  gen- 
eral use.  Objective  methods  for  this  purpose  are  numer- 
ous, and  several  optical  firms  manufacture  a  millimeter 
gauge,  sometimes  with  a  vernier  (Fig.  150),  there  being  two 
projecting  points  to  measure  the  distance  between  the  cen- 
ters of  the  pupils.  For  the  work  of  an  optician  such  a  gauge 
is  very  convenient.  The  difficulty  with  all  of  these  is  that 
the  observer  does  not  know  whether  his  own  eye  is  exactly 
opposite  the  eye  observed,  and  any  parallax  is  therefore  not 
taken  into  account.  In  order  to  obviate  this  difficulty,  I 
constructed  several  years  ago  a  simple  arrangement  (B  482) 


FIG.  151. — Visuometer  of  the  author. 

shown  in  front  view,  and  in  section,  in  Fig.  151.  It  consists 
of  two  millimeter  scales,  parallel  to  each  other,  held  firmly 
just  below  the  level  of  the  eyes  by  means  of  a  head-band. 
In  order  to  reduce  still  farther  any  error  from  parallax  there 
are  two  "  sights "  which  project  about  fifty  millimeters  in 
front  of  the  anterior  scale.  The  illustrations  show  the  con- 
struction at  a  glance.  When  using  the  instrument,  the  ob- 
server slides  the  double  scale  along  the  horizontal  slot  until 
the  zero  point  with  the  corresponding  stationary  projecting 
"  sight  "  is  opposite  the  center  of  the  pupil  of  the  right  eye 
of  the  person  under  examination.  The  left  projecting  sight, 
which  moves  in  a  horizontal  slot  of  its  own  in  the  anterior 


The  Interocular  Base  Line  217 

plate,  is  then  slid  along  its  bar  until  that  sight  is  opposite 
the  center  of  the  pupil  of  the  left  observed  eye.  The  exact 
adjustment  of  these  two  sights  often  requires  a  little  care, 
but  when  accomplished,  it  remains  only  to  read  off  on  the 
scale  the  distance  between  them,  and  this  is  the  base  line. 
For  ordinary  purposes  it  is  unnecessary  to  bind  the  visuome- 
ter  on  the  head.  Certainly,  for  prescribing  glasses  correctly, 
all  the  surgeon  needs  to  do  is  to  hold  the  visuometer  with 
his  left  hand  against  the  forehead  of  the  patient,  adjust  the 
sights  with  the  right  hand,  and  in  a  moment  read  off  the 
distance  between  the  centers.  The  arrangement  which  Hess 
(B  483)  has  devised  to  measure  the  base  line  (Fig.  152)  con- 
sists essentially  of  a  scale  placed  before  the  observed  eyes 
and  two  small  sights,  over  which  the  person  under  examina- 
tion looks  at  a  distant  object.  The  observer  reads  off  the 
length  of  the  base  line  on  the  scale.  While  this  is  simple  in 
theory  it  is  not  always  easy  to  manipulate  exactly,  but  is 
nevertheless  one  of  the  best  and  most  convenient  arrange- 
ments for  the  purpose. 


FIG.  152. — Visuometer  of  Hess. 

When  special  exactness  is  desired  for  laboratory  purposes, 
I  have  found  (B  482)  that  the  base  line  can  be  measured  con- 
veniently as  follows.  The  person  under  examination  fixes  the 
head  in  the  head-rest,  and  to  its  forehead-piece  a  millimeter 
measure  is  attached  horizontally.  This  millimeter  measure 
and  the  eyes  are  then  viewed  through  a  small  telescope  which 
has  a  micrometer  eyepiece.  By  moving  the  telescope  back 
and  forward,  a  point  is  found  at  which  a  certain  number  of 
millimeters  (for  example,  three)  of  the  measure  which  is 
attached  to  the  head-piece  just  coincides  with  a  certain 


218  The  Interocular  Base  Line 

number  of  divisions  (for  example,  one)  of  the  micrometer 
scale. 

A  candle  or  small  lamp  is  placed  a  meter  or  more  in  front 
of  the  subject,  and  at  such  an  angle  as  to  illumine  the  eyes 
under  examination  and  also  the  micrometer  scale  of  the  tele- 
scope. The  patient  then  fixes  some  distant  object  straight 
in  front.  As  the  observer  looks  through  the  telescope  at 
the  eyes,  he  sees  on  each  cornea  a  minute  point  of  light  re- 
fleeted  from  the  candle.  The  distance  between  these  two 
points  is  then  counted  off  on  the  micrometer  scale  in  the 
eye  piece  and  the  corresponding  distance  in  millimeters  is 


FIG.  153. — Telescope  visuometer  of  the  author. 

known.  The  arrangement  is  shown  by  Fig.  153.  As  the 
telescope  stands  in  the  dark  room,  its  place  on  the  table  being 
marked  so  that  it  is  always  at  the  right  distance  from  the 
head-rest,  one  can  measure  the  base  line  in  this  way  very 
quickly  and  easily. 

The  importance  of  some  such  measurement  will  appear 
later.  It  must  suffice  here  to  repeat  that  a  knowledge  of  the 
length  of  the  base  line  is  essential  in  the  accurate  measure- 
ment of  relative  accommodation,  and  a  convenience  in 


Forms  of  Heterophoria  219 

determining  the  distance  between  the  centers  of  glasses, 
especially  when  they  are  unusually  strong. 

§  3.  Forms  and  Nature  of  Heterophoria. — The  earlier 
writers  on  this  subject  were  accustomed  to  say  that  the  eyes 
are  "  at  rest  "  or  are  "  in  a  static  condition  "  when  they  are  in 
the  "primary  position."  This  "  primary  position,"  as  we  know, 
is  when  the  visual  axes  lying  in  the  horizontal  plane  are  par- 
allel to  each  other  and  perpendicular  to  the  line  joining  the 
centers  of  the  two  eyes.  It  is  impossible  here  to  discuss  any 
of  the  numerous  theories  relating  to  this  subject.  Suffice 
it  to  say  that  the  studies  of  Edmund  Hansen  Grut  (B  507) 
have  been  an  important  factor  in  shaping  ophthalmological 
opinions  concerning  the  position  of  rest  and  the  causes  of 
what  we  call  strabismus.  His  view  that  the  eyes  naturally 
tend  to  diverge  has  been  adopted  by  many  English  writers, 
and  followed  closely  by  Landolt  and  Galezowski,  but  in 
American  and  German  literature  we  find  comparatively  little 
about  it.  Indeed,  in  this  country  we  had,  until  lately,  held 
to  the  earlier  idea  that  the  primary  position  is  also  the  posi- 
tion of  rest.  Fortunately,  during  recent  years  we  have 
improved  our  methods  of  examination,  and  therefore  ap- 
proach the  question  better  equipped  than  before. 

But  if  we  attempt  to  make  use  of  these  later  methods  of 
examination  or  tests  of  the  muscle  imbalance  which  are  so 
constantly  employed  in  practice,  we  must  first  decide  whether 
the  tests  themselves  are  of  any  value,  and  if  so,  which  are 
the  most  reliable ;  also  what  r61e  the  eyes  themselves  play 
in  any  such  examinations.  In  most  text-books  these  "tests" 
are  described  in  the  chapter  on  heterophoria — that  is,  in 
connection  with  pathological  conditions.  But  before  we  can 
judge  intelligently  of  the  importance  of  any  such  measure- 
ments of  diseased  eyes,  we  should  try  these  same  tests 
upon  normal  eyes.  A  physiological  standard  is  what  we  need. 
It  is  also  what  we  lack.  We  must  agree  now  upon  that 
standard,  even  though  it  necessitates  a  considerable  digres- 
sion. In  any  such  examination  it  is  assumed : 

A.  That  the  subject  is  of  average  intelligence ; 

B.  That  he  has  binocular  vision  ;  and 

C.  That  the  head  can  be  placed   in  exactly  the  same 
position  whenever  desired. 


220  Forms  of  Heterophoria 

The  problem  before  us  is  to  ascertain  the  position  of  the 
visual,  and  also  of  the  vertical  or  the  horizontal  axes  in 
the  position  which  we  call  "  rest."  For  the  present  we  will 
consider  that  the  word  rest  means  simply  a  relaxation,  to  as 
great  an  extent  as  possible,  of  the  extraocular  muscles,  al- 
though we  shall  find  that  this  is  only  an  apparent  rest.  How- 
ever that  may  be,  the  fact  is  that  eyes  which  are  perfectly 
normal  otherwise,  and  which  never  gave  their  owners  any 
inconvenience,  when  placed  in  the  position  of  apparent  or 
of  actual  rest  often  tend  to  deviate  from  the  primary  posi- 
tion into  positions  more  or  less  abnormal.  It  is  therefore 
proper  at  this  point  to  recall  at  least  a  few  of  the  terms 
which  describe  these  positions  (B  725). 

Orthophoria,  is  usually  described  as  the  "tendency  of  the 
visual  axes  in  parallelism." 

Heterophoria,  the  tendency  of  the  eyes  to  turn  in  any 
other  direction. 

Esophoria,1  the  tendency  of  one  or  both  eyes  to  turn 
inward. 

Exophoria,  the  tendency  of  one  or  both  eyes  to  turn 
outward. 

Hyperphoria,  the  tendency  of  one  eye  only  to  turn  upward. 

Anophoria,  the  tendency  of  both  eyes  to  turn  upward. 

Hypophoria,  the  tendency  of  one  eye  only  to  turn  down- 
ward. 

Katophoria,  the  tendency  of  both  eyes  to  turn  downward. 

Cyclophoria,  the  tendency  of  one  or  both  vertical  axes  to 
revolve  in  or  out  about  the  antero-posterior  axis. 

Besides  glancing  thus  at  the  different  forms  of  heteropho- 
ria,  it  is  essential  to  recall  also  their  real  nature.  We  should 
keep  in  mind  the  fact  that  they  are  all  essentially  passive 
conditions.  In  order  to  know  whether  or  not  heterophoria 
exists,  it  is  necessary,  in  all  of  our  tests,  first  to  dissociate 
the  retinal  image  in  one  eye  from  that  in  the  fellow  eye,  and 
having  thus  removed  all  tendencies  to  single  vision,  each 
globe  swings,  or  tends  to  swing,  into  the  position  most  natural 
to  it.  In  other  words,  every  form  of  heterophoria  requires 
the  constant  effort  of  one  or  more  groups  of  the  muscles,  that 

1  Esophoria,  not  eesophoria,  as  it  is  sometimes  pronounced. 


The  Environment  221 

is,  a  corresponding  duction,  to  overcome  it,  as  long  as  single 
vision  is  maintained.  If  that  effort  is  excessive  or  insuffi- 
cient, or  if  not  in  accord  with  certain  fundamental  principles 
which  we  shall  see  control  muscle  balance,  then  it  gives  rise  to 
a  large  group  of  annoying  symptoms.  Their  character  will 
be  studied  later  from  the  pathological  standpoint. 

In  any  attempt  to  ascertain  the  position  of  the  visual  axes 
by  means  of  appliances  or  tests,  we  have  to  do  with  two 
factors.  One  of  these  is  the  instrument  used,  with  its  con- 
comitants, and  the  other  is  the  eye  of  the  individual.  It  will 
be  found  convenient  to  consider  each  of  these  factors  in 
order. 

§  4.  Precautions  Necessary  with  All  Tests — The 
Environment. — In  considering  the  instruments  now  at  our 
command  for  this  purpose,  we  have  to  do  with  the  appli- 
ance itself,  and  also  with  the  environment — that  is,  the  room, 
the  test  light,  etc.  It  may  seem  elementary  to  refer  to  such 
simple  matters,  but  a  variation  in  these  details  certainly 
causes  a  variation  in  the  results,  and  as  the  chief  object  here 
is  to  eliminate  confusion,  the  methods  employed  must  be  as 
nearly  uniform  as  possible.  As  these  concomitants  are  the 
same  for  all  the  tests,  it  is  easier  to  consider  them  first. 

(A)  The  Room.  Results  perceptibly  more  constant  can  be 
obtained  if  the  tests  are  made  in  a  darkened  room  six 
meters  or  more  in  length.  Evidently  such  rooms  are  not 
obtainable  by  the  average  practitioner  in  a  crowded  city,  and 
are  to  be  found  only  as  parts  of  an  ophthalmological  labora- 
tory. But  this  means,  practically,  that  when  replies  in  the 
consulting  room  are  contradictory,  particularly  those  of  an 
apparently  stupid  subject,  the  answers  will  often  be  less  con- 
fusing if  the  same  tests  are  made  in  the  same  room  at  night. 

Thus  we  find  that  the  Maddox  rod,  an  admirable  test  in 
many  respects,  is  practically  useless,  even  for  an  intelligent 
patient,  if  the  room  is  so  bright  as  not  to  allow  the  streak 
to  be  plainly  visible,  or  if  there  are  adjacent  to  the  test  light, 
bright  points  from  which  that  light  itself  is  also  reflected, 
and  thus,  by  giving  several  streaks  instead  of  one,  confuse 
the  patient  into  contradictory  replies.  It  is  desirable  at 
least  to  have  a  board  about  a  meter  square  or  a  space  of  that 


222 


The  Test  Light 


size  behind  the  test  light  covered  with  black  cloth  or  paper, 
in  order  to  avoid  points  of  reflection. 

(B)  The  Test  Light.     As  most   of   these   tests   are  made 
with  a  light  placed  at  a  distance  of  six  meters  or  more,  we 
should  understand  what  kind  of  a  light  is  referred  to.       We 

are  accustomed  to  think  of  a  candle 
flame  as  the  most  convenient,  espe- 
cially as  that  is  the  light  usually  fig- 
ured. The  fact  is,  however,  that  this 
is  inconvenient  and  inaccurate.  Not 
only  does  the  flame  bend  from  side 
to  side  with  each  draft  of  wind, 
but  it  varies  in  height.  This  is  es- 
pecially annoying  when  exact  tests 
are  desired  with  regard  to  the  tend- 
ency of  the  eye  to  turn  up  or  down. 
For  that  reason,  if  a  candle  is  used, 
the  flame  should  be  enclosed  in  an 
opaque  cylinder  like  that  which  Landolt 
proposed,  having  a  circular  opening  on 
one  side.  The  best  form  is  that  in 
which  a  gas  or  an  electric  light  is 
screened  behind  a  circular  opening, — as, 
for  example,  in  the  Thorington  chimney 

(Fig.  154). 

(C)  The  Head-Rest. —  For  routine  office  work  with  ordi- 
nary patients,  it  is  sufficient  to  instruct  the  person  to   hold 
the  head  erect  and  still,  but  when  the  most  exact  results  are 
desired  in  physiological  studies,  or  with  restless    patients, 
especially  children,  it  is  more  satisfactory  to  use  the  head- 
rest already  described  (B  553-) 

As  we  sometimes  wish  special  accuracy  when  using  prisms 
singly  or  in  the  form  of  the  phorometer,  the  latter  instrument 
has  been  attached  to  the  head -rest,  as  in  Fig.  155.  The 
arrangement  is  such  as  to  permit  adjustment  of  the  phoro- 
meter to  the  height  of  the  eyes,  or  to  allow  it  to  be 
swung  entirely  away  from  the  face,  when  it  interferes  at 
all  with  changes  of  glasses  or  with  other  manipulations. 


FlG.  154. — Thorington's 
chimnev. 


The  Head-Rest 


223 


Doubtless  many  an  ophthalmologist  may  insist  that  such 
precautions  regarding  the  room,  the  light,  and  the  adjust- 
ment of  the  head,  etc.,  are  entirely  unnecessary. 


FIG.  155. — Head-rest  of  the  author  with  photometer  attachment. 

It  is  quite  true  that  very  fair  clinical  work  can  be  accom- 
plished by  the  cruder  method  of  placing  a  candle  in  the 
distance  and  holding  a  prism  in  the  hand  before  the  eye  of 
the  person  under  examination,  but  as  one  of  the  main  objects 
of  this  study  is  to  obtain  results  as  constant  as  possible,  it 
becomes  an  evident  necessity  to  employ  exact  methods. 

Laboratory  procedure  is  sometimes  quite  different  from 
that  of  the  consulting  room,  but  after  practice  with  the 
former  it  is  unconsciously  followed  as  part  of  the  clinical 
routine,  to  the  satisfaction  of  the  practitioner  and  the  advan- 
tage of  the  patient. 


DIVISION  II. 
Tests  to  Determine  the  Position  of  Rest  of  the  Visual  Axes. 

§  i.    Classification  of  the  Tests.  —    First  Group.  — 

We  assume  that  we  are  able  to  determine  the  position  of  rest 
of  the  visual  axes  by  means  of  tests  which  depend  on  the 
fact  that,  when  the  tendency  to  binocular  vision  has  in 
any  way,  been  abolished,  each  eye  actually  does  swing,  or  at 
least  tends  to  swing,  into  that,  position  which  it  can  occupy 
with  the  least  amount  of  effort.  For  the  present  we  will 
grant  that  assumption,  although  a  little  later  we  shall  see 
that  this  tendency  is  subject  to  various  modifications.  The 
tests  employed  are  the  witnesses,  as  it  were,  in  the  case  be- 
fore us,  and  upon  their  evidence  we  must  decide.  Let  us, 
therefore,  bring  them  before  us,  and  note  the  nature  of  the 
evidence  furnished  by  each.  After  that,  we  can  observe 
whether  any  witness  contradicts  itself  at  different  times,  or 
whether  the  witnesses  contradict  each  other.  Usually  stu- 
dents appreciate  better  the  uses  of  the  tests  for  heterophoria 
if  they  are  arranged  in  three  groups.  Disregarding  the  chro- 
nological order  in  which  they  were  described,  and  omitting 
some  of  their  modifications,  we  can  arrange  these  tests,  ac- 
cording to  their  action,  in  three  groups.  Thus  we  have : 

First,  a  group  in  which  the  retinal  image  in  each  eye  re- 
mains clear,  but  one  or  both  of  these  images  is  displaced  from 
the  macula  by  means  of  a  prism  with  the  base  up  or  down,  be- 
fore one  eye  or  both. 

Here  we  have : 

A.  The  single  prism,  base  up  or  down,  as  first  suggested 
by  Graefe. 

B.  That  modification  of  the  single  prism  which  we  know 
as  the  phorometers  of  Stevens,  Savage,  and  others. 

(  A  )     The  prism  or  prisms,  base  up  or  down. 
Graefe  applied  the  principle  here  involved  to  a  vertical  line 
drawn  through  a  dot,  viewed  at  the  near  point.     But  as  that 

224 


Tests  with  a  Vertical  Prism 


225 


involved  the  acts  of  both  accommodation  arid  convergence, 
and  as  we  are  now  studying  both  eyes  at  rest,  evidently  the 
test  object  must  be  situated  six  meters  distant.  For  this 
purpose  the  test  object  is  the  bright  circular  opening  in  an 
opaque  chimney  already  described.  With  all  these  methods 
the  test  depends  on  the  use  of  one  or  two  prisms,  and 
this  method,  even  though  familiar  to  most  readers,  should 
be  described  if  only  for  the  sake  of  completeness. 

When,  for  example,  a  prism  is  held  base  down  before  the 
right  eye,  as  the  observer  looks  at  a  distant  light,  the  image 
of  this  light  falls  on  the  lower  part  of  the  retina  of  that  eye. 


FIG.   156. —  Positions   of  the  retinal  images  as  seen  from  behind   in  or- 
thophoria. 

If  orthophoria  is  present,  the  retinal  image  of  the  right  eye  is 
displaced  from  F  to  F'  (Fig.  156),  and  the  light  seen  through 
the  prism  with  that  eye  is  straight  above  the  one  seen  di- 
rectly with  the  left  eye.  But  if  the  right  eye  tends  to  turn 
inward  (Fig.  157),  then  the  image  of  the  light  falling  on  the 


FIG.  157. — Positions  of  the  retinal  images  as  seen  from  behind  in  esophoria. 
lower  and  inner  portion  of  the  retina  F'  causes  the  light  to 
appear  above  and  to  the  right  of  the  real  one  seen  with  the 
left  eye.  Moreover,  the  amount  or  the  number  of  degrees 
which  the  right  eye  turns  inward  is  then  shown  by  the 
strength  of  a  second  prism  held  with  it's  base  outward,  in 
front  of  the  first  prism,  so  as  to  deflect  the  image  outward 

15 


226  Tests  with  a  Vertical  Prism 

sufficiently  to  fall  directly  below  the  macula  of  that  right 
eye.  In  other  words,  this  horizontal  prism  changes  the  po- 
sition of  the  retinal  image  of  the  right  eye  from  F'  to  F".  The 
two  images  of  the  light  then  appear  in  a  vertical  line.  In  a 
similar  manner,  if  the  right  eye  tends  to  turn  outward,  the 
image  of  the  light  falling  in  the  lower  and  outer  portion  of 
the  retina  seems  to  be  above  and  to  the  left  of  the  real  light. 
The  amount  which  the  eye  turns  outward  is  shown  by  the 
strength  of  a  second  prism,  base  inward,  which  brings  the 
two  lights  into  a  vertical  line. 

It  is  easy  to  see  that  this  experiment  could  be  varied  in 
many  ways.  One  of  these  is  to  arrange  a  scale  for  the  speedy 
and  accurate  determination  of  the  position  of  the  eyes,  such 
as  has  been  constructed  by  Herbert  (B  533).  This  scale 
consists  of  a  black  background  with  several  white  squares  to 
the  right,  left,  up,  and  down  respectively  from  a  central  square, 
thus  forming  a  cross,  each  spot  being  one  centimeter  square 
and  so  arranged  that  the  separation  of  each  square  is 


FIG.  158. — Herbert's  arrangement  of  dots  for  testing  and  measuring  heter- 
ophoria. 

equivalent  to  a  deflection   of   one  prism    diopter.      If   the 
squares  are  to  be  viewed  at  a  distance  of  five  meters,  they 


Tests  with  a  Vertical  Prism 


227 


must  be  drawn  five  centimeters  from  center  to  center. 
If,  however,  that  distance  from  the  scale  to  the  patient 
is  not  convenient,  any  other  may  be  chosen,  the  rule 
being  that  for  every  decrease  of  one  meter,  the  distance 
between  the  squares  must  be  decreased  one  centimeter, 
and  for  every  increase  of  one  meter  the  squares  must  be 
separated  one  centimeter.  Figure  158  illustrates  the  arrange- 
ments of  the  dots  or  small  squares.  White  squares  can 
be  cemented  on  black,  or  black  ones  on  a  white  back- 


FIG.    159. — Same  arrangement  in  orthophoria  with  a  prism  of  five  degrees 
base  down  before  right  eye. 

ground.  The  scale  should  be  hung  up  in  a  good  light, 
opposite  a  window  if  possible,  the  center  of  the  scale 
being  on  the  same  level  as  the  patient's  eyes.  By  placing 
a  5-degree  prism  base  down  in  front  of  one  eye,  the 
cross  will  be  deflected  upward  five  squares,  and  if  ortho- 
phoria is  present  there  will  be  two  horizontal  arms,  as  shown 
in  Fig.  159.  But  if  esophoria  is  present,  and  we  place  a 
prism  base  down  before  the  right  eye,  then  the  second  cross 


228  Tests  with  a  Vertical  Prism 

is  seen  above  and  to  the  right  (Fig.  160),  or  with  exophoria 
it  would  be  seen  above  and  to  the  left. 


,     •',  3      1      s     6 


FlG.  160. — Same  arrangement  in  esophoria. 

(B)  That  modification  of  the  single  prism  which  we  know 
as  the  phorometer  of  Stevens  (B  514)  (Fig.  161),  depends  on 
the  same  principle,  except  that  two  prisms  placed  opposite 
each  other  are  used,  one  with  the  base  down,  for  example,  be- 
fore one  eye,  and  another,  with  the  base  up,  before  the  other. 
As  these  prisms  are  revolved  in  opposite  directions,  the  images 
of  a  distant  dot  or  point  of  light  change  their  relative  positions, 
and  the  degree  of  the  heterophoria,  if  any  is  thus  revealed, 
is  read  off  on  a  scale  calculated  for  that  purpose.    Instead  of 
placing  one  prism  before  each  eye,  Risley  (B  508  )  obtains  the 
same  effects  by  mounting  both  prisms  together  in   such  a 
way  that    they   revolve    on    each  other,    both    before    one 
eye.     The  principle  involved  is  similar,  as  we  see  at  a  glance, 
to  that  of  the  Stokes  lens.      This  form  of  the  prism  is  con- 
venient and  has  decided    advantages,   but   is  not   quite  as 
exact  as  the  more  elaborate  arrangement  of  Stevens.       The 
same  may  be  said  also  of  others  of  this  type. 

(C)  The  double  prism  of  Maddox  (Fig.  162)  consists  of 
two  prisms,  each  of  about  four  to  six  degrees,  joined  at  their 


Tests  with  the  Double  Prism 


229 


FIG.    161. — Phorotneter  of  Stevens 


FlG.    162. — Double  prism  of  Maddux. 


230 


Tests  with  the  Glass  Rod 


bases.    When  these  are  held  before  the  right  eye,  for  exam- 
ple, with  the  line  of  their  bases  horizontal,  the  upper  prism 


FIG.    163. — Action  of  the  double  prism. 

deflects  the  image  of  the  distant  test  light  upon  the  lower 
part  of  the  retina,  and  the  lower  one  deflects  another  image 
of  the  light  upon  the  upper  part  of  the  same  retina  (Fig.  163.) 
In  this  way  the  observer  sees  three  lights.  If  or- 
thophoria  is  present,  they  are  in  the  same  vertical  line,  and 
the  central  one  (seen  with  the  uncovered  eye)  is  half  way  be- 
tween the  other  two.  If  the  prism  is  held  thus  before  the 
right  eye,  in  esophoria  the  middle  light  of  course  appears  to 
the  left  of  the  other  two,  in  exophoria  to  the  right.  In  right 
hyperphoria,  it  appears  nearer  the  lower  light.  In  right 
hypophona,  nearer  the  upper  light,  etc. 

§  2.  In  the  Second  Group  the  retinal  image  falls  on 
the  macula  in  each  eye — in  one  with  the  usual  clearness,  but  in 
the  other  with  the  image  so  distorted  or  blurred  or  changed  in 
color  as  to  abolish  all  effort  at  fusion. 


FIG.    164. — Glass  rod  of  Maddox,  simple  form. 


Tests  with  the  Glass  Rod 


231 


FIG.  165. — Compound  glass  rod  of  Maddox. 

As  the  best  types  of  this  group  we  have  : 
(A.)  The  glass  rod  of  Maddox,  simple  or  compound,  in  its 
various  forms. 

(B.)  The  stenopaic  lens. 
(C.)  The  cobalt  glass. 


FIG.  166. 


FIG.  167. 


FIG.  168. 


Tests  of  the  static  position  when  a  glass  rod  is  held  horizontally  before 
the  right  eye  and  the  person  under  examination  looks  at  the  round  opening  in 
the  chimney.  (166)  Appearance  seen  in  orthophoria  ;  (167)  in  esophoria  ; 
(  1 68  )  in  exophoria. 


232 


Tests  with  the  Glass  Rod 


A.  The  glass  rod  of  Maddox.  Although  this  device  is  so 
familiar  to  every  practitioner,  the  principle  which  it  involves 
must  be  referred  to.  When  this  glass  rod  in  the  simpler  form 
(Fig.  164),  or  in  series  (  Fig.  165  ),  is  held  horizontally  before 
the  right  eye,  for  example,  while  the  observer  looks  at  a 
distant  candle  or  round  spot  of  light,  a  bright  vertical  streak 
is  produced  on  the  retina.  If  the  visual  axes  are  parallel,  this 
vertical  line  appears  to  pass  through  the  spot  of  light  (Fig. 
166).  If  the  eye  tends  to  deviate  inward,  the  image  of  the 
bright  line  falls  on  the  inner  portion  of  the  retina  and  appears 
to  be  on  the  right  side  of  the  light  (Fig.  167).  If  the 
eye  tends  to  deviate  outward,  the  vertical  streak  of  light  falls 
on  the  outer  part  of  the  retina  and  appears  to  be  on  the  left 
side  of  the  light  (Fig.  168).  If  the  rod  is  held  vertically, 
the  streak  of  light  will  be  horizontal. 


FIG.  169.  FIG.  170.  FIG.  171. 

Tests  of  the  static  position  when  a  glass  rod  is  held  vertically  before  the 
right  eye  and  the  person  under  examination  looks  at  the  round  opening  in  the 
chimney.  (  169)  Orthophoria  ;  (  1 70 )  right  hyperphoria  ;  (171 )  right  hypo- 
phoria. 


Tests  with  the  Stenopaic  Lens 


233 


If  the  visual  axes  of  the  two  eyes  are  in  the  same  horizontal 
plane,  the  horizontal  streak  of  light  will  pass  through  the 
center  of  the  test  light  (Fig.  169).  If  the  axis  of  the  right  eye 
tends  to  turn  upward,  the  streak  will  appear  below  the  light 
(Fig.  170),  or  if  the  axis  tends  to  turn  downward,  the  streak 
will  appear  above  the  light  (  Fig.  171  ). 

B.  The  stenopaic  lens  (B  515  ).  When  a  lens  of  small 
diameter  (Fig.  172  )  and  of  12  or  13  diopters  focal  length  is 
held  before  the  right  eye,  for  example,  and  the  observer 


FIG.  172. —  The  stenopaic  lens  (Stevens).  / 


looks  at  a  distant  light,  it  produces  on  the  retina  a  blurred 
image  of  the  distant  candle  or  point  of  light.  If  the  visual 
axes  are  parallel,  the  light  appears  to  be  in  the  center  of  the 
blurred  spot  (  Fig.  173  C),  but  if  the  right  eye  turns  outward, 
the  image  of  the  blurred  disc,  being  then  projected  inward, 
the  light  appears  to  be  nearer  its  right  edge,  or  if  it  turns  out 
and  downward,  the  blurred  image  being  projected  up  and  to 


234 


Tests  with  the  Cobalt  Glass 


Orthophoria  Heterophoria. 

FIG.   173.  — Action  of  the  stenopaic  lens  when  the  axes  are  (C)  parallel  (Z?) 
not  parallel. 

the  left,  the  light  appears  to  be  near  its  lower  and  right-hand 

edge  (Fig.  173 /?  ). 

C.      The   cobalt  glass.     If   a 

piece  of  colored  glass  (Fig.  174) 
preferably  of  a  dark  cobalt  blue, 
of  such  thickness  as  to  allow 
only  a  little  light  to  pass  through 
it,  is  held  before  one  eye  while 
the  observer  looks  at  a  distant 
candle  or  spot  of  light,  he  sees 
only  an  indistinct  image  of  the 
light.  In  other  words,  this  acts 
like  thestenopaic lens, and  tests 
made  with  the  blurred  spot  of  col- 
ored lightare  similarin  every  way 
FIG.  174— Cobalt  glass  (Ridgeway).  to  tests  made  with  such  a  lens. 

§  3.  A  Third  Group  of  tests  includes  those  in  which, 
one  eye  being  covered  with  the  hand  or  by  a  screen,  and  thus 
excluded  entirely  from  the  visual  act,  it  swings  into  the  posi- 
tion most  natural  to  it.  When  deviation  occurs  under  such 
circumstances,  we  have,  subjectively,  the  so-called  parallax 
test.  Then,  by  quickly  uncovering  the  excluded  eye,  there 
is  diplopia  for  an  instant,  and  an  apparent  movement  of  the 
test  object  as  single  vision  is  re-established.  Also  we  have, 


The  Cover  Test  235 

objectively,  displacement  or  a  momentary  malposition  of  the 
eye  of  the  subject,  which  the  surgeon  sees  just  at  the 
instant  when  the  hand  or  card  is  withdrawn. 

With  some  clinicians  this  is  a  favorite  test  (B  553-565). 
It  is  true  that  the  more  intelligent  patients  will  recognize 
the  displacement  which  the  object  presents  at  the  instant 
when  the  card  which  covers  one  eye  is  removed.  But 
although  this  test  is  veiy  simple  in  theory  it  often  requires 
skill  in  both  patient  and  surgeon.  Many  of  the  former, 
especially  those  who  are  uneducated  or  naturally  dull,  are 
utterly  incapable  of  estimating  the  distance  which  seems  to 
exist  between  the  real  light  and  the  one  which  appears  to  be 
displaced  at  the  instant  when  one  eye  is  uncovered.  This 
is  especially  true  when  the  vision  of  both  eyes  is  not  quite 
the  same.  This  cover  test,  therefore,  apparently  cannot  be 
considered  one  upon  which  to  rely,  either  objectively  or 
subjectively. 

The  diploscope  of  Remy,  however,  deserves  mention.  The 
principle  upon  which  it  is  based  is  similar  to  the  cover  test,  and 
can  be  understood  better  by  a  simple  experiment  in  parallax. 
If  the  index  finger  be  held  vertically  and  at  arm's  length  in 
front  of  the  face,  while  the  observer  looks  with  both  eyes 
at  a  series  of  four  lezters, — for  example,  K  O  B  A —  arranged 
in  a  horizontal  line  across  the  room,  it  is  possible  to  place 
these  letters  at  such  a  distance  from  each  other  and  from  the 
observer  that,  with  his  visual  axes  parallel,  he  can,  by  closing 
his  left  eye,  see  only  K  B,  and  by  closing  the  right  he  can  see 
only  O  A.  If,  however,  by  placing  suitable  prisms  before  the 
eyes,  a  tendency  for  the  visual  axis  to  turn  inward  or  outward 
is  created,  or  if  esophoria  or  exophoria  exist,  then  with  the 
letters  and  with  the  finger  remaining  in  the  same  position  he 
may  read  K  B  A  or  K  O  A,  or  other  combinations  depending 
upon  the  position  of  the  visual  axes. 

Now  the  essential  part  of  the  diploscope  also  consists  of  a 
screen  similar  to  the  finger  in  the  median  line.  In  the  earlier 
form  of  the  instrument  it  was  a  vertical  strip  of  brass,  a  little 
wider  than  a  finger  and  about  as  long,  which  could  be  placed 
vertically  or  turned  horizontally  as  desired.  Later  a  disc  of 
brass  about  eight  centimeters  in  diameter  was  perforated 


236 


The  Cover  Test 


with  two  circular  openings  (Fig.  175)  each  about  twenty 
millimeters  in  diameter,  the  distance  between  the  adjacent 
edges  of  these  openings  being  about  fifty-five  millimeters,  or 


FIG.    175. — Diploscope  of  Remy. 

a  little  less  than  the  distance  between  the  centers  of  the  eyes 
of  the  average  individual.  Therefore  the  screen  of  brass  be- 
tween these  openings  serves  the  same  purpose  as  the  inter- 
cepting finger  in  the  simple  experiment  just  referred  to.  This 
circular  disc  is  also  perforated  in  its  vertical  diameter  with 
two  other  circular  openings,  of  about  the  same  size  and 
same  distance  from  each  other  as  are  the  horizontal  openings 
before  mentioned.  The  vertical  openings  are  for  testing 
vertical  deviations  of  the  eyes.  A  strip  of  thin  brass,  which 
is  a  little  broader  than  the  diameter  of  these  circular  openings, 
is  attached  at  its  center  to  the  center  of  the  disc.  By  turn- 
ing this  strip  it  can  be  made  to  cover  the  two  vertical  or  the 
two  horizontal  openings  as  desired. 

While  this  simple  disc  is  the  essential  part  of  the  diplo- 
scope,  for  convenience  and  accuracy,  something  depends 
upon  its  being  properly  mounted,  so  as  to  place  it  and  keep 
it  at  the  proper  distance  from  the  eye.  In  order  to  shut  out 
extraneous  rays,  this  disc  is  placed  at  the  farther  end  of  a 
tube  of  brass  some  twenty  or  thirty  centimeters  long,  the 
proximate  end  being  left  open.  A  square  brass  rod  about 
a  meter  in  length  is  attached  near  its  center  to  the  lower 


Comparison  of  Tests  237 

side  of  this  cylinder.  At  the  distal  end  of  the  rod  there  is 
a  rack  which  holds  the  test  letters,  and  at  the  proximate  end 
there  is  a  short  step  in  the  bar  upon  which  the  chin  of  the 
person  can  rest  when  the  tests  are  made,  thus  keeping  the 
eyes  always  at  the  same  distance  approximately  from  the 
screen  and  of  course  from  the  letters.  The  entire  instru- 
ment is  mounted  on  an  adjustable  foot-piece  to  give  it  sta- 
bility, and  the  upright  arranged  so  that  it  can  be  adapted 
to  the  position  and  height  of  the  individual. 

This  brief  glance  at  the  structure  of  the  diploscope  is  suf- 
ficient for  our  purpose.  Its  use  is  indicated  by  the  simple 
experiment  in  parallax  already  cited.  The  fact  that  a  per- 
son can  see  with  the  instrument  the  four  letters  K  O  B  A  or 
three  of  them,  or  two,  indicates,  as  before  mentioned,  the 
position  of  the  eyes.  The  truth  is  that,  after  having  tested 
this  method  quite  thoroughly,  it  has  seemed  to  me  at  least, 
one  of  the  least  convenient  or  exact  with  which  to  determine 
the  static  position  of  the  eyes,  although  some  practitioners, 
rely  upon  it  almost  entirely.  It  is,  however,  exceedingly 
useful  in  showing  the  presence  of  binocular  vision,  and 
especially  in  detecting  cases  of  malingering. 

§  4.  Which  of  these  Methods  is  the  Best  for  Determin- 
ing the  Latent  Position  of  the  Visual  Axis?  —  In  view  of 
the  number  of  instruments  and  methods  and  modifications 
of  both  which  are  constantly  proposed,  we  naturally  ask 
which  of  all  is  the  best.  The  confusion  of  opinions  among 
clinicians  on  this  point  shows  how  essential  it  is  to  secure 
some  approach  to  uniformity.  In  order  to  decide  this,  we 
must  agree  upon  certain  criteria  by  which  the  excellence  of 
one  method  or  another  can  be  judged. 

First,  the  instruments  must  be  so  constructed  that  they 
can  be  arranged  at  all  times  in  exactly  the  same  position,  and 
the  readings  made  with  the  same  degree  of  accuracy.  If  we 
examine  the  instruments  in  turn,  we  see  that  they  do  not 
comply  equally  with  that  demand.  When  the  single  prism  is 
square  and  is  held  in  a  square  frame,  its  position  can  always 
be  promptly  and  quite  accurately  adjusted.  Stevens'  phor- 
ometer  in  some  form  is  probably  the  best  instrument  we  have 
considered  from  this  point  of  view.  With  the  spirit-level, 


238  Comparison  of  Tests 

the  base  of  the  prism  can  easily  be  placed  horizontally,  and 
if  the  index  is  properly  constructed,  the  readings  are  easy 
and  always  accurate. 

Second,  the  retinal  image  which  the  test  object  forms  must 
be  of  such  a  kind  as  to  produce  no  confusion  in  the  results, 
or  at  least  the  minimum  amount  possible. 

This  requisite  has  to  do  with  the  perceptive  power  of  the 
retina,  as  well  as  with  the  test  itself.  The  influence  of  the 
muscles,  retina,  and  brain  upon  the  apparent  static  condition 
of  the  eye  will  be  considered  later.  But  just  at  present  let  us 
confine  ourselves  to  the  question  of  the  degree  in  which  the 
different  tests  comply  with  this  second  demand.  Here 
again  the  tests  of  the  first  group  are  quite  satisfactory,  as  are 
those  of  the  second  group  also. 

When  the  streak  of  the  glass  rod  is  clearly  visible,  the  exact 
vertical  or  horizontal  position  of  the  streak  is  readily  recog- 
nized. As  for  the  stenopaic  lens  or  the  cobalt  glass,  the  nature 
of  these  instruments  is  such  that  they  readily  conform  to  the 
first  requirement,  though  they  do  not  to  the  second. 

The  tests  of  the  third  class  evidently  do  not  fulfil  the  first 
requirement.  Of  course  it  is  as  easy  to  exclude  one  eye  at 
one  time,  as  at  another,  but  it  is  impossible  to  measure  accu- 
rately either  the  parallax,  which  is  observed  by  the  patient 
or  the  displacement,  which  is  observed  by  the  surgeon. 

Without  going  into  further  details  here,  it  may  be  stated 
in  general  that  tests  of  the  first  group  tend  to  give  more 
constant  results  than  those  of  the  second  group,  and  these, 
more  constant  results  than  those  of  the  third  group. 

From  the  foregoing,  therefore,  it  is  evident  that  the  tests 
here  described  are  by  no  means  all  of  equal  clinical  value, 
and  an  arrangement  according  to  their  order  of  excellence 
would  be  apparently  about  as  follows : 

1st.     Stevens'  phorometer. 

2d.     Graefe's  prism,  when  accurately  placed  before  the  eye. 

3d.     The  compound  colored  Maddox  rod. 

4th.  The  stenopaic  lens,  the  double  prism,  and  the  col- 
ored glass. 

Jth.    The  cover  test  either  subjectively  or  objectively. 

Such  a*n  arrangement  is  of  very  decided  practical  import- 


Other  Factors  in  the  Statics  of  the  Eyes      239 

ance.  It  is  true  that  the  relative  value  of  some  of  these  tests 
has  been  discussed  by  Stevens  (3514),  Hubbell  (B  548), 
and  others,  but  much  greater  exactness  is  necessary  before 
we  can  lessen  materially  the  existing  confusion  concerning 
muscular  statics.  We  must  have  a  scale  of  values  for  these 
tests,  such  as  is  given  here,  or  one  more  accurate,  as  de- 
termined by  future  experiments.  The  text-books  should  not 
give  to  each  test  the  same  apparent  value,  as  is  usually  done, 
but  should  make  clear  the  usefulness  of  some,  and  the  use- 
lessness  of  others.  Finally,  when  a  writer  makes  any  state- 
ment relating  to  muscular  statics  he  should  also  make  it  a 
rule  to  specify,  with  other  details,  which  test  or  tests  have 
been  employed.  Until  this  is  done,  our  present  confusion 
must  continue. 

§  5.  What  is  the  Influence  of  the  Eyes  in  Determining 
the  Static  Position  of  the  Visual  Axes  ? — The  foregoing 
tests,  alone  or  together,  constitute  one  factor  in  determining 
the  position  which  the  visual  axes  assume  or  tend  to  assume. 
The  second  factor  is  the  influence  exerted  by  certain  parts 
or  functions  of  the  eyes.  These  are: 

(  A  )  The  ciliary  muscles  with  the  corresponding  function 
of  accommodation.  It  is  often  taken  for  granted  that  a  pair 
of  eyes  is  entirely  "  at  rest "  when  they  are  in  the  primary 
position.  That,  we  know,  is  not  necessarily  the  case,  for  no 
one  can  say  what  is  the  condition  of  the  ciliary  muscle  with- 
out also  measuring  the  refraction.  In  cases  of  hypermetro- 
pia,  some  tension  of  that  muscle  of  course  is  necessary  if  the 
person  sees  distant  objects  clearly. 

Again,  a  familiar  experience  indicates  that  the  eyes  are  not 
necessarily  "  at  rest  "  when  they  do  assume  the  primary  posi- 
tion. When  we  use  almost  any  one  of  the  test  instruments  of 
the  first  or  second  class — for  example,  the  glass  rod, —  we 
notice  that  as  the  subject  looks  at  the  distant  light  the  vertical 
streak  does  not  remain  stationary.  Frequently  this  streak 
apparently  swings  several  times  from  side  to  side.  But  as 
the  rod  is  stationary,  and  as  the  head  is  also  stationary — or 
should  be  —  evidently  that  motion  of  the  streak  must  be  due 
to  the  action  of  the  recti  muscles.  But  as  the  recti  muscles 
in  turn  are  intimately  associated  with,  and  partly  dependent 


240  Rest  is  Apparent  or  Actual 

upon  the  ciliary  muscles,  the  latter  in  reality  thus  influence 
very  decidedly  the  static  position  of  the  visual  axes. 

The  influence  of  the  ciliary  muscle  in  determining  the  static 
position  of  the  visual  axes  is  also  shown  by  the  fact  that  with- 
out atropin  the  foregoing  tests,  when  made  on  certain  indi- 
viduals, give  more  or  less  conflicting  results,  whereas  after  a 
sufficient  use  of  atropin  these  results  tend  to  greater  uni- 
formity. 

(B)  The  recti  muscles  influence  the  static  condition  of 
the  visual  axes.     In  certain  cases,  after  a  cycloplegic  has  been 
used  and  a  perfect  correction  of  the  ametropia  made,  if  the 
rod  be  placed  before  one  eye  horizontally,  the  patient  will 
still  assert  that  the  vertical  streak  moves  from  side  to  side 
before  it  assumes  a  position  of  rest.     Evidently  this  must  be 
due  to  an  action  of  the  recti  muscles  alone.     There  are  other 
facts  which  point  in  the  same  direction,  but  this  simple  ex- 
periment is  apparently  conclusive. 

(C)  The  condition  of  the  retina  influences  the  static  posi- 
tion of  the  visual  axes,  through  the  ability  or  inability  of  the 
individual  to  perceive  a  displaced  or  blurred  image.    Certain 
persons  find  difficulty  in  recognizing  such  an   image,  even 
though  it  is  clearly  formed  ;    and  occasionally  one  is  met 
with  who  cannot  distinguish  it  at  all,  even  though  he  be  in- 
telligent and  the  retina  normal,  as  far  as  can  be  ascertained. 

§  6.  The  so-called  "  Position  of  Rest  "  is  either  APPARENT 
or  ACTUAL.  As  the  action  of  the  ciliary  muscles,  either 
alone  or  in  conjunction  with  the  recti,  constitutes  a  large 
factor  in  the  tendency  to  error  in  these  tests,  it  seems  desir- 
able, from  the  standpoint  of  the  physiologist  and  especially 
from  that  of  the  clinician,  to  separate  the  condition  which 
we  call  "  rest  "  into  two  forms.  The  first,  or  apparent  rest, 
is  simply  a  relaxation,  more  or  less  complete,  of  the  extraoc- 
ular  muscles.  The  second,  or  actual  rest,  is  relaxation  not 
only  of  the  extraocular,  but  also  of  the  intraocular  muscles. 
It  is  true  the  difference  is  usually  not  great,  and  in  most  cases 
can  be  disregarded,  but  when  muscle  imbalance  persists  for  a 
long  time  and  in  any  annoying  degree  this  difference  should 
certainly  be  taken  into  account  as  a  part  of  the  diagnosis  and 
therefore  as  a  factor  in  the  treatment.  Until  that  difference 


Usual  Position  of  the  Visual  Axes 


241 


is  recognized,  the  clinician  does  not  always  know  what  he  is 
measuring,  no  matter  how  exact  may  be  the  instruments  or 
his  method  of  using  them.  Also,  when  uncertain  results 
are  recorded  they  are  confusing  to  himself  and  to  others. 
The  practical  importance  of  this  point  is  too  evident  to  re- 
quire any  elaboration. 

§  7.  What  then  is  the  Usual  Position  of  the  Visual 
Axes  when  Normal  Eyes  are  in  a  State  of  Apparent 
Rest  ? 

The  first  reliable  tests  were  made  by  Bannister  upon  the 
eyes  of  one  hundred  soldiers  at  Fort  Leaven  worth  in  1897 
(B  579).  At  present  we  will  consider  only  what  appeared 
to  be  the  static  condition.  The  results  which  he  obtained 
were  striking  and  instructive,  in  showing  that  orthophoria" 
was  by  no  means  necessary  to  comfortable  vision,  as  had 
formerly  been  supposed.  Partly  to  verify  these  figures  and 
to  exclude  sources  of  error,  tests  were  made  of  the  static  con- 
dition of  the  visual  axes  among  other  soldiers  stationed  at 
Fort  Porter,  who  were  being  examined  partly  with  reference 
to  this  and  also  as  to  other  phases  of  their  muscular  condi- 
tion. The  results  obtained  by  Bannister,  those  which  I 
obtained,  and  the  result  of  a  third  group  (of  Harvard  stu- 
dents) who  were  measured  by  Dr.  Chas.  H.  Williams  and 
myself,  are  found  in  the  following  table. 


4 

« 

M 

ja 

o 

O 

a 
^-  o 

Examiner 

No. 

Occupation 

°"'z 

S^ 

a. 

S^ 

a. 

fc? 

rt  -5 

£^ 

'^J 

o 

^  'r™1 

O 

W 

W 

u  i) 

>Q 

Bannister 

IOO 

Soldiers 

60 

60 

29 

29 

7 

7 

7 

7 

Howe 

5«> 

Soldiers 

22 

39 

18 

32 

9 

16 

7 

12 

Williams   ) 
and  Howe  ) 

3- 

Students 

12 

33 

10 

32 

4 

12 

5 

16 

IS? 

94 

50 

57 

30 

20 

IO 

19 

IO 

Quite  recently  this  question  was  investigated  by  Biel- 
schowsky  and  Ludwig  (B  580),  their  object  being  to  ascertain 
not  simply  the  percentage  of  heterophoria  as  compared  with 
orthophoria,  but  also  the  percentage  of  heterophoria  in 


242  Conclusion  as  to  Position  of  Rest 

healthy  individuals  as  compared  with  that  which  exists  among 
neurotics  and  also  among  those  who  have  what  is  commonly 
called  muscular  asthenopia.  These  findings  will  be  considered 
in  detail  when  studying  the  pathology  of  the  muscles.  It  is 
sufficient  in  this  connection  to  state  that  these  two  observ- 
ers also  found  heterophoria  much  more  common  in  normal 
eyes  than  is  orthophoria,  and  it  may  be  added  that,  accord- 
ing to  them,  heterophoria  exists  quite  as  frequently  in  the 
normal  as  it  does  in  the  abnormal  conditions  just  mentioned. 

It  will  be  observed  that  the  percentages  found  by  different 
examiners  differ  somewhat  from  each  other.  I  have  taken 
pains,  however,  to  inquire  by  letter  of  both  Bannister  and 
Bielschowsky  as  to  the  probable  causes  of  this,  and  it  seems 
altogether  probable  that  such  differences  are  due  in  part 
to  the  different  methods  of  making  the  examinations. 

From  the  data  thus  far  obtained  we  must  conclude  con- 
cerning the  apparent  position  of  rest. 

1st.  Orthophoria  is  not  present  in  the  majority  of  normal 
eyes. 

2d.  Esophoria  is  almost  as  common  as  orthophoria, 
exophoria  is  less  common,  and  vertical  variations  probably 
the  least  common  of  all. 

§  8.  The  Conclusions  Regarding  the  Static  Position 
of  the  Visual  Axes  are  : 

1st.  The  methods  ordinarily  used  for  determining  the 
static  position  of  the  visual  axes  are  not  altogether  satis- 
factory. This  is  because  : 

a.  Different  tests  in  constant  use  are  of  different  value ; 
and 

6.  Certain  parts  or  functions  of  the  eyes  themselves  in- 
fluence the  position  of  the  axes. 

2d.  In  order  to  obviate  these  difficulties  and  to  ascertain 
as  nearly  as  possible  the  actual  static  position  of  the  visual 
axes,  it  is  desirable  to  have  : 

(A.)     A  suitable  room  in  which  to  make  the  examinations. 

(B.)     A  proper  test  light. 

(C.)  To  employ  always  the  same  test  in  exactly  the  same 
way. 

(D.)     If  extreme  exactness  is  necessary,  it  is  desirable  to 


Position  of  the  Vertical  Axes  243 

have  the  eyes  of  the  subject  under  the  full  effect  of  a  cyclo- 
plegic. 

(E.)     To  correct  any  existing  ametropia. 

3d.  Unless  these  precautions  are  taken,  we  do  not  ascer- 
tain the  actual  but  only  the  apparent  static  position  of  the 
visual  axes. 

4th.  As  the  apparent  static  position  of  the  visual  axes  is 
quite  variable,  being  dependent  on  different  factors,  it  is 
evident  that  any  conclusions  as  to  diagnosis  or  treatment 
based  on  that  rinding  alone  are  also  inexact. 

5th.  It  would  tend  to  accuracy  if  an  Ophthalmological 
Congress  would  agree  upon  uniform  methods  of  testing  and 
of  expressing  the  results  of  such  tests  of  the  static  position  of 
the  visual  axes.  Until  more  scientific  and  uniform  methods 
are  adopted,  we  must  continue  to  have  confusion  in  diag- 
nosis and  treatment. 


DIVISION  III. 

§  i.  Tests  to  Determine  the  Position  of  the  Vertical 
Axes.  Does  the  Eye  Rotate  on  its  Antero-Posterior 
Axis  to  Reach  a  Position  of  Rest  ? — In  our  study  of  the 
eyes  at  rest,  we  have  considered  thus  far  only  those  tendencies 
(forms  of  heterophoria)  in  which  the  visual  axes  turn  in,  out, 
up,  down,  or  obliquely.  It  is  evident,  however,  that  there 
may  also  exist  a  tendency  for  the  vertical  axes  to  revolve  about 
the  antero-posterior  diameter  in  order  to  assume  a  position  of 
most  complete  rest.  The  position  of  the  vertical  axes  when 
the  visual  axes  are  parallel,  and  also  when  they  converge,  was 
long  ago  investigated  by  Volkmann  (B  581),  Helmholtz 
(B  584),  Hering  (B  586),  and  several  others.  It  might 
suffice  in  one  way  for  our  present  purpose  to  make  the  sim- 
ple statement  that  when  the  eyes  are  in  the  primary  position 
the  vertical  axes  tend  to  diverge  upward  at  an  angle  with 
each  other  of  about  three  degrees. 

As  this  angle  is  so  small  as  to  be  hardly  perceptible  by 
most  measurements,  it  may  seem  unworthy  of  much  atten- 
tion— indeed,  in  itself  it  is  of  no  clinical  importance.  But  the 
fact  that  the  vertical  axes  do  thus  revolve  about  the  antero- 
posterior  axes  brings  us  to  one  phase  of  that  important  func- 
tion which  we  call  true  torsion.  It  will  save  much  confusion 
therefore  and  considerable  repetition,  if  at  this  point  we 
glance  at  the  methods  by  which  it  is  possible  to  determine 
the  position  of  the  vertical  axes  when  the  visual  axes  are  in 
the  primary  position.  Most  of  the  earlier  students  of  this 
subject  depended  in  part  upon  the  fusion  of  two  similar 
retinal  images  which  were  vertical  or  nearly  vertical.  The 
tendency  of  the  muscles  to  fuse  such  images  will  be  con- 
sidered in  detail  later.  For  our  present  purpose  we  must 
consider  certain  tests  which  dissociate  the  retinal  images 
entirely  or  at  least  in  part,  thus  allowing  each  globe  to  rotate 
into  the  position  most  natural  to  it.  Most  of  the  tests  of  this 

244 


Tests  of  Cyclophoria  245 

kind  for  determining  the  position  of  the  vertical  axes  have 
come  into  use  comparatively  recently.  Therefore  a  disregard 
of  the  chronological  order  of  the  description  of  the  tests  for 
determining  the  position  of  the  vertical  axes  enables  us  to 
group  them,  as  we  have  already  grouped  different  tests  for 
determining  the  position  of  the  visual  axes  when  the  eyes 


B 


FIG.  176 —  Appearance  presented  by  forms  of  heterophoria  when  a  double 
prism,  bases  horizontal,  is  held  before  the  right  eye  and  the  observer  views  a 
distant  horizontal  line.  '(A.)  Orthophoria  ;  (  B )  tipping  of  the  vertical  axes 
inward  ;  (C)  tipping  of  the  vertical  axes  outward. 

are  in  a  state  of  apparent  or  actual  rest.  Indeed  the  groups 
are  in  a  certain  way  similar  to  each  other.     Thus  we  have : 

§  2.  First  a  group  in  which  the  retinal  image  in  each  eye 
remains  clear  but  one  of  these  images  is  displaced  from  the 


246  Tests  of  Cyclophoria 

macula  by  means  of  a  double  prism.  This  has  been  described 
(Chap.  III-Div.  II -Sec.  i),  and  we  have  seen  how  its  use 
with  a  distant  test  light  enables  us  to  detect  latent  devia- 
tions of  the  visual  axes.  The  same  prism  enables  us  to  detect 
also  latent  deviations  of  the  vertical  axes.  For  this  purpose 
we  use  a  horizontal  line  as  a  test  object.  For  example,  if 
such  a  prism  be  held  before  the  right  eye  while  the  individ- 
ual looks  at  a  horizontal  line,  if  orthophoria  is  present,  then 
when  the  left  eye  is  also  open  the  observer  sees  three  lines 
which  are  parallel  to  each  other  (Fig.  176  A),  the  one 
viewed  with  the  left  eye  being  half  way  between  the  two  pro- 
duced by  the  prism.  But  if  there  is  a  tendency  for  the  left 
eye,  for  example,  to  revolve  about  its  antero-posterior  axis, 
this  third  line,  which  belongs  to  the  left  eye,  is  no  longer 
horizontal,  but  it  is  tipped  obliquely  in  one  direction  or  the 
other.  (Fig.  176  B  and  C  ).  This  subject  has  been  elabo- 
rated by  Savage  ( B  462 )  in  a  manner  quite  familiar  to 
American  readers. 

Various  slight  modifications  of  this  test  have  been  sug- 
gested, but  they  depend  upon  the  same  principle  or  are  so 
nearly  identical  with  it  that  they  need  not  be  dwelt  upon 
here. 

§  3.  The  Second  Group  of  Tests  includes  those  in 
which  the  image  of  the  test  object  falls  unchanged  on  the  ma- 
cula and  a  part  of  the  retina  of  one  eye,  and  the  image  of 
another  test  object  falls  unchanged  upon  the  macula  and 
another  part  of  the  retina  of  the  other  eye. — These  test  objects 
are  usually  in  principle  at  least,  some  form  of  the  Volkmann 
discs.  Their  forms  will  be  referred  to  more  in  detail  a  little 
later.  At  this  point  it  is  sufficient  to  say  that  he  was  one  of  the 
earlier  students  of  this  question  and  suggested  measuring  the 
position  of  the  vertical  axes  by  means  of  vertical  radii  (not 
with  diameters)  drawn  on  two  circular  pieces  of  card- 
board (B.  581 ).  At  present  we  will  confine  our  attention  to 
these  two  discs  a  single  radius  on  one  being  directed  upward, 
and  on  the  other,  downward  (Fig.  177).  These  discs  are  of 
such  a  size  that  the  distance  between  their  centers  can  be 
made  to  correspond  to  the  distance  between  the  centers  of 
the  eyes  under  examination.  By  turning  one  or  both  of 


Form  of  Clinoscope 


247 


the  discs  a  few  degrees  in  or  out,  it  is  possible  to  determine 
at  what  point  the  two  radii  seem  to  form  a  single  vertical  line. 
Since  the  time  of  Volkmann,  the  same  experiments  have 
been  repeated  by  many  others  arid  several  appliances  based 
on  this  principle  have  resulted. 


EAMEYBOWnr.NX 


FIG.  177. —  Volkmann's  discs,  simple  form. 

(A)  Stevens'  Clinoscope.— Stevens  (B  598)  placed  one  of  these 
discs  at  each  end  of  a  tube  and  called  the  arrangement  a 


FIG.  178. — Adaptation  of  Volkmann's  discs  to  the  clinoscope  of  Stevens. 

clinoscope  (Fig.  178).     The  most  recent  model  of  this  consists 
essentially   of  two   brass     tubes    each    about   twenty  cen- 


248  Another  Form  of  Clinoscope 

timeters  long  and  two  or  more  centimeters  in  diameter. 
At  the  distal  end  of  each  tube  there  is  a  Volkmann's  disc 
(tipped  sometimes  for  convenience ")  and  at  the  proximal  end 
a  small  opening,  the  distance  between  the  tubes  being  adjust- 
able to  correspond  to  the  distances  between  the  centers  of 
the  pupils.  The  illustration  shows  at  a  glance  the  excellent 
mechanical  arrangement  for  levelling  the  instrument,  for 
measuring  the  amount  which  each  disc  is  revolved  around 
the  axis  of  the  tube,  and  the  amount  which  the  .tubes  are 
elevated,  depressed,  etc.  Stevens'  clinoscope  is  not  adapted, 
however,  for  measuring  the  tipping  outward  of  the  vertical 
axes  in  the  act  of  convergence  for  the  reason  that  the  tubes 
cannot  be  made  to  converge  sufficiently. 

(B)  Another  Form  of  the  Clinoscope. — We  will  see  later 
that  the  principal  use  of  an  instrument  of  this  kind  in  clinical 
work  is  not  for  determining  the  position  of  the  vertical 
axes  when  the  eyes  are  at  rest — the  point  which  we  have  now 
under  consideration, — but  it  is  rather  to  determine  the  posi- 
tion which  those  axes  assume  when  the  visual  axes  converge. 
It  therefore  seemed  desirable  to  arrange  the  discs  so  that  we 
could  measure  with  them  the  position  of  the  vertical  axes 
when  the  eyes  are  in  the  primary  position  and  also  when  the 
visual  axes  converge.  This  was  accomplished  by  taking 
away  -the  tubes  entirely,  placing  the  discs  directly  before  the 
eyes,  as  Volkmann  did,  and  then  providing  a  simple  but  ac- 
curate means  by  which  the  discs  could  be  set  at  any  desired 
distance  from  each  other  or  from  the  observer.  The  form 
which  seemed  to  serve  this  purpose  best  is,  in  substance,  as 
follows  (Fig.  179). 

First.  A  head-rest  ( F  F' ).  This  is  »a  modification 
of  the  Helmholtz  bit  which  has  been  already  described 

Second.  A  median  bar  (H  H')  which  rests  at  each  end  upon 
an  upright  and  whose  height  can  be  adjusted  as  desired. 

Third.  A  transverse  bar  (  B  B' )  which  measures  on  sec- 
tion about  five  millimeters  square  and  which,  being  attached  to 
the  carrier  T  slides  along  the  median  bar  H  H'.  The  trans- 
verse bar  has,  on  each  side,  a  small  carrier,  supporting  a 
metal  frame.  In  the  figure  this  is  obscured  by  the  Volk- 
mann discs.  In  front  of  the  lower  half  of  this  metal  frame 


Usual  Position  of  the  Vertical  Axes 


249 


there  is  a  grooved  arc,  and  in  this  a  disc(V  and  V7) 
rests.  The  upper  part  of  the  metal  frame  supports  a 
graduated  arc  (D  D').  An  index  attached  to  each  disc 
indicates  on  this  graduated  arc  the  exact  position  of  the 
radii. 


FlG.  179. — Adaptation  of  Volkmann  s   discs   to  the  converging  clinoscope 
of  the  author. 

Fourth.  A  second  carrier  (  P  )  on  the  median  bar  bears  a 
small  projecting  point  whose  height  can  be  raised  or  lowered 
as  desired  by  a  screw  thread  and  its  upper  portion  is  deeply 
indented  by  a  narrow  notch.  The  object  of  this  projecting 
point  is  to  hold  a  card  or  other  screen  in  the  median  line, 
when  the  Volkmann  discs  are  viewed  with  the  eyes  in  the 
primary  position.  Or,  if  the  instrument  is  used  to  measure 
the  position  of  the  vertical  axes  when  the  visual  axes  con- 


250  Usual  Position  of  the  Vertical  Axes 

verge,  then  this  projecting  point  serves  as  a  point  of  fixation. 
There  are  other  details  which  assist  in  these  measurements 
of  convergence,  but  they  need  not  detain  us  in  this  connec- 
tion. Let  us  suppose  for  our  present  purpose  that  we  wish  to 
measure  the  amount  of  tipping  of  the  vertical  axes  which 
occurs  when  the  visual  axes  are  in  the  primary  position.  To 
do  that,  the  carrier  T  is  pushed  toward  the  distal  end  of  the 
median  bar  at  H' ;  the  discs  (V  V')  are  then  brought  near 
to  the  bar  so  that  the  distance  between  their  centers  is  the 
same  as  the  distance  between  the  centers  of  the  eyes.  Some 
persons,  at  least  with  a  little  practice,  can  then  dissociate 
the  retinal  images  in  such  a  way  that  the  right  eye  sees  only 
the  disc  (V)  and  the  left  one  the  disc  (V).  Ordinarily, 
however,  it  is  necessary  to  fix  a  card  as  a  screen  in  the  median 
plane  in  such  a  way  that  each  eye  is  prevented  from  seeing 
the  disc  which  is  opposite  the  other  eye.  In  that  way  the 
images  of  the  discs  can  be  blended  and  the  instrument  thus 
serves  the  same  purpose  as  the  Stevens  clinoscope  when  that 
is  adjusted  to  measure  the  position  of  the  vertical  axes  with 
the  eyes  in  the  primary  position. 

§  4.  What  is  the  Usual  Position  of  the  Vertical 
Axes  when  the  Visual  Axes  are  in  the  Primary  Posi- 
tion ? — The  foregoing  descriptions  of  the  different  methods 
of  measurement  have  been  given  not  only  that  we  may 
know  how  these  measurements  are  made,  but  also  how 
various  degrees  of  cyclophoria  can  be  recognized.  The  ques- 
tion still  remains,  however,  what  is  the  normal  position  of  each 
vertical  axis  ?  It  may  therefore  be  repeated  that  the  physio- 
logical vertical  axes  are  not  absolutely  vertical,  but  when  the 
eyes  are  in  the  primary  position  these  axes  are  inclined  down- 
ward  toward  each  other,  at  an  angle  of  from  about  three  to 
four  degrees  —  on  the  average,  about  three  and  one-third 
degrees.  It  is  unnecessary  to  go  into  detail  concerning 
slightly  different  results  obtained  by  different  observers  and 
the  probable  causes  of  these  differences.  Suffice  it  to  say  that 
measurements  made  more  recently  agree  in  general  with 
those  by  the  first  observers  already  mentioned.  This 
inclination  of  the  vertical  axes  from  the  median  plane  is  so 
slight  that  it  need  not  usually  be  taken  into  consideration. 


What  is  Cyclophoria?  251 

Indeed,  while  understanding  that  the  verticals  are  thus  in- 
clined to  each  other,  it  is  customary  to  speak  of  them  as  if  they 
were  actually  vertical.  This  will  therefore  be  done  in  what 
follows  except  when  special  exactness  is  desired. 

§  5.  Which  Test  for  Cyclophoria  is  the  Best?  — 
Having  thus  glanced  at  a  few  of  the  different  tests  for  cyclo- 
phoria,  the  question  arises,  which  is  the  best  either  for 
laboratory  measurement  or  for  clinical  work?  It  is  unnec- 
essary to  review  the  various  criteria  by  which  any  such  tests  are 
to  be  judged.  Suffice  it  to  say  that  as  the  double  prism  is  the 
simplest  and  the  one  most  readily  understood,  it  commends 
itself  most  for  clinical  work.  It  is  also  the  one  upon  which 
practitioners  usually  depend  if  they  take  the  trouble  to  make 
any  tests  of  this  kind.  It  is,  however,  by  no  means  exact. 
It  simply  indicates,  in  a  general  way,  what  the  condition  is,  and 
although  an  approximation  to  the  degree  can  be  obtained  by 
placing  a  proper  prism  before  the  other  eye,  that  method  is 
also  inexact. 

For  all  reliable  measurements  and  physiological  studies, 
some  form  of  the  clinoscope  is  essential.  Office  patients, 
after  a  little  practice  with  the  instrument,  usually  give  intelli- 
gent replies  as  to  the  position  of  the  lines,  and  such  measure- 
ments are  of  course  infinitely  more  satisfactory  than  can  be 
obtained  with  the  double  prism.  But  these  tests  with 
ignorant  or  stupid  patients  result  only  in  confusion  and 
vexation  of  spirit. 

§  6.  Exactly  what  is  Cyclophoria  ? — Since  normal  eyes 
in  a  state  of  rest  frequently  tend  to  assume  some  other  than 
the  primary  position,  and  since  these  variations  from  the 
normal  type  occur  also  in  pathological  conditions,  it  was 
necessary  to  recall  the  terms  describing  these  conditions.  In 
other  words,  it  was  necessary  to  define  the  various  forms  of 
heterophoria.  Cyclophoria  was  then  defined  as  a  tendency  of 
the  vertical  axes  to  turn  in  or  outward  from  the  vertical  me- 
ridian. That  definition  is  usually  sufficiently  exact,  but  we 
have  just  seen  that  under  normal  conditions  what  we  call  the 
vertical  axes  are  not  in  reality  quite  vertical,  but  that  the  up- 
per end  of  each  axis  tips  outward  at  an  angle  of  from  one  and 
a  half  to  two  degrees  from  the  median  plane.  In  a  strict 


252  What  is  Cyclophoria? 

sense,  therefore,  this  slight  deviation  must  be  taken  into  ac- 
count. If  in  using  the  clinoscope  we  find  it  necessary  to  tip 
the  upper  radius  out  four  degrees  before  the  two  radii  appear 
to  form  a  single  vertical  line,  we  cannot  say  strictly  that  there 
is  a  tendency  for  each  axis  to  turn  out  four  degrees,  but  only 
two,  or  two  and  a  half ;  or  when  the  two  radii,  being  placed  in 
an  exact  vertical  line,  seem  to  form  a  vertical  line  there  ex- 
ists in  reality  a  slight  degree  of  cyclophoria.  In  other  words, 
in  the  measurement  of  this  we  must  allow  first  for  the  nor- 
mal outward  tipping  of  each  vertical  axis  of  one  and  a  half  to 
two  degrees,  and  then  count  any  variation  from  that  point  as 
real  cyclophoria. 

In  this  connection  it  is  necessary  to  clear  up  if  possible 
some  of  the  confusion  existing  in  regard  to  the  terms  plus 
and  minus  cyclophoria.  Several  American  writers  define 
the  term  plus  cyclophoria  as  a  tendency  of  the  upper  end  of 
the  vertical  axes  to  turn  inward  toward  the  vertical  plane  or 
beyond  it,  and  minus  cyclophoria,  as  a  tendency  to  turn 
outward.  The  confusion  produced  by  this  definition  is  par- 
ticularly unfortunate.  For,  as  the  upper  end  of  the  axis  does 
tend  to  turn  outward,  as  we  shall  see,  in  the  act  of  converg- 
ence, any  motion  in  that  direction  is  naturally  indicated  by 
the  plus  sign.  If  these  terms  are  to  be  retained  at  all,  plus 
should  indicate  the  turning  of  the  upper  end  of  the  vertical 
axis  outward  from  the  normal  position,  and  minus,  a  turning 
inward.  To  avoid  ambiguity,  however,  it  is  better  to  use  the 
words  outward  and  inward  from  the  vertical  or  from  the  nor- 
mal position  when  that  is  specified. 

The  clinical  importance  of  cyclophoria  might  be  dwelt 
upon  here,  for  as  it  is  eminently  a  passive  condition,  the 
ocular  muscles  must  make  a  corresponding  effort  to  turn  the 
vertical  axes  into  the  position  which  they  normally  oc- 
cupy. In  other  words,  a  given  degree  of  cyclophoria  must 
always  be  accompanied  by  a  corresponding  effort  of  torsion. 
Any  discussion  of  the  clinical  importance  of  torsion  is,  how- 
ever, deferred  until  we  study  another  aspect  of  the  subject. 


CHAPTER  V. 

BOTH  EYES  IN  MOTION  ;  AND   FIRST  GROUP  OF 
ASSOCIATED   MOVEMENTS. 

§  i.  Definition  and  Mechanism  of  Associated  Move- 
ments.— Our  study  of  the  motions  of  one  eye  alone  has  in- 
cluded much  which  relates  also  to  the  motions  of  both  eyes. 
Thus,  what  we  know  concerning  the  action  of  one  or  more 
muscles  moving  one  eye,  applies  as  well  to  both  eyes.  This 
is  also  true  of  the  rapidity  of  the  lateral  motions,  of  the  field 
of  fixation,  etc.,  so  that  our  study  of  the  motions  of  both 
eyes  is  rather  a  study  of  their  action  together,  or  what  is 
known  as  associated  movements.  In  the  introduction  to 
his  monograph  on  binocular  vision,  Hering(B  250)  says  that 
the  two  eyes  may  be  regarded  as  the  halves  of  a  single  organ. 
To  the  student  of  the  ocular  muscles  this  means  that  in  order 
to  avoid  double  vision,  each  eye  acting  with  its  fellow  eye 
instinctively  turns  its  visual  axis  to  the  point  to  which  the 
attention  is  directed  at  that  moment. 

But  by  what  mechanism  can  we  explain  the  motor  impulses 
which  rotate  both  eyes  in  the  same  direction  at  the  same 
time,  as  when  we  look  from  right  to  left,  or  again  in  opposite 
directions,  as  in  convergence?  Innumerable  theories  have 
been  offered  to  account  for  this  and,  as  usual,  their  number 
was  in  proportion  to  our  ignorance  of  the  subject.  Gradu- 
ally they  have  been  discarded,  however,  as  our  knowledge  of 
the  functions  of  the  cells  in  the  nuclei  and  in  different  portions 
of  the  brain  has  grown  more  exact.  At  present  there  is  con- 
siderable unanimity  of  opinion  on  the  general  propositions 
concerning  the  motor  impulses  and  the  routes  by  which 
they  are  sent  from  the  gyrus  angularis,  or  from  the  nuclei, 
to  the  eyes.  The  present  condition  of  our  knowledge  is 

253 


254        Mechanism  of  Associated  Movements. 


shown  in  Bernheimer's  (B  185)  diagrammatic  representation 
(Fig.  181).     From  this  it  appears  that  when  a  motor  impulse 


Lfye 


Extern^/ 
Reclus 


Nucleus  of  the] 
Abc/ucens      } 


FIG.  181. — Diagrammatic  representation  of  the  cells  which  preside  over  lateral 
motions  of  the  eyes  and  the  fibers  conducting  the  motor  impulses  (Bernheimer). 

originates  in  the  gyms  angularis  on  the  right  side,  for  ex- 
ample, that  impulse  crosses  to  the  left  side  of  the  brain,  com- 
municating with  the  nucleus  of  the  abducens  and  thence 
through  to  the  external  rectus  of  the  left  eye,  while  at  the 
same  time  a  part  of  the  same  impulse  passes  through  the 
nucleus  of  the  third  nerve,  and  thence  to  the  internal  rectus 
of  the  right  eye,  causing  both  eyes  to  turn  toward  the  left. 
Or  an  impulse  for  convergence  may  originate  from  the  gyrus 


Classification  of  Associated  Movements.      255 

angularis  on  the  right  side,  and  passing  through  the  nucleus 
of  the  sixth  may  be  continued  to  the  nucleus  of  the  third 
nerve,  and  from  that  point  to  each  internal  rectus.  Or  con- 
vergence may  occuf  through  an  impulse  originating  in  the 
cells  which  lie  in  the  center  of  the  nucleus  of  the  third  nerve. 
Undoubtedly  this  explanation  of  associated  movements  will 
be  modified  by  further  studies,  but  it  accounts  for  the  ob- 
served facts,  perhaps  better  than  any  other.  We  do  not  know 
the  details  of  the  mechanism  by  which  any  of  these  associ- 
ated movements  are  produced,  but  such  facts  as  we  have, 
tend  to  show  that  there  are  at  least  six  conjugate  innerva- 
tions,  one  for  each  of  the  associated  movements, — down,  up, 
to  the  right,  to  the  left,  for  convergence,  and  for  torsion. 
These  might  be  called  the  principal  innervations.  In  addi- 
tion, there  are  certainly  several  others — no  one  can  say  how 
many — to  rotate  the  eye  obliquely  in  various  directions. 

§  2.  Classification  of  Associated  Movements. — These 
can  be  conveniently  arranged  in  four  groups. 

1st.  The  parallel  visual  axes  being  in  the  primary  posi- 
tion, the  upper  end  of  each  vertical  axis  revolves  about 
the  visual  axis '  to  the  right  or  left,  or  they  turn  at  the 
same  time  medianward  or  temporalward. 

2d.  The  parallel  visual  axes  move  in  one  of  the  prin- 
cipal meridians  to  the  right,  left,  up,  or  down.  In  this  group 
of  movements  there  is  no  torsion. 

3d.  The  parallel  visual  axes  move  obliquely.  In  this 
group  the  vertical  and  horizontal  axes  appear  to  change  their 
position  in  such  a  manner  as  to  produce  "  false  torsion." 

4th.  The  visual  axes  do  not  remain  parallel,  but  converge 
toward  each  other.  In  so  doing,  the  upper  end  of  each  ver- 
tical axis  rotates  slightly  outward  producing  "  true  torsion." 

Strictly  speaking,  it  is  often  neither  the  optic  nor  the  vis- 
ual axis  about  which  a  torsional  movement  is  made,  but  one 
usually  called  the  antero-posterior  axis.  For  convenience, 
however,  this  being  understood,  it  is  better  to  speak  only  of 
the  visual  axis  as  the  hub  of  the  wheel  motions.  This  outline  of 

1  As  we  can  easily  measure  the  angle  alpha,  the  optic  axis  may  be  considered 
as  practically  the  same  as  the  visual  axis. 


256       First  Group  of  Associated  Movements. 

the  classification  of  the  different  associated  movements  indi- 
cates the  plan  to  be  pursued  in  their  study.  Each  of  these 
groups  of  movements  is  of  importance  in  its  way,  and  it  is 
only  for  clearness  of  description  that  this  sequence  is  fol- 
lowed. 

§  3.  First  Group  of  Associated  Movements. — Defini- 
tion. The  parallel  visual  axes  being  in  the  primary  position, 
the  upper  end  of  each  vertical  axis  revolves  about  the  visual 
axis  to  the  right  or  left,  or  they  turn  at  the  same  time 
medianward  or  temporalward.  These  motions  are  very  lim- 
ited and  probably  not  of  much  importance  clinically,  but  as 
the  principles  on  which  they  depend. must  be  considered  in 
connection  with  the  true  torsion  which  accompanies  conver- 
gence, it  is  well  to  understand  what  they  are  and  how  they 
are  measured.  At  the  outset  we  should  appreciate  clearly 
that  what  we  are  dealing  with  now  is  not  cyclophoria,  but  a 
cycloduction.  In  order  to  determine  the  former,  we  found 
it  was  necessary  first  to  dissociate  the  retinal  images  and  then 
allow  each  vertical  axis  to  rotate  into  the  position  most  nat- 
ural to  it.  Cyclophoria,  like  all  the  other  phorias,  is  essen- 
tially passive,  while  the  group  of  associated  movements  now 
to  be  considered  are  essentially  active  movements.  They 
depend  on  the  instinctive  desire  to  fuse  images  which  are  not 
already  entirely  dissociated,  but  which  fall  on  parts  of  the 
two  retinas  so  nearly  corresponding  to  each  other  that  the 
muscles  try  to  turn  the  globe  so  a?  to  overcome,  if  possible, 
any  double  vision.  Now  when  t.ne  muscles  thus  rotate  the 
vertical  (and  horizontal)  axes  about  the  antero-posterior 
axes  in  the  interest  of  single  vision,  we  have  a  torsion  or,  as 
this  is  also  called,  a  circumduction  or  a  cycloduction.  In  a 
strict  sense,  the  word  "  torsion  "  does  not  indicate  whether  the 
turning  is  of  a  passive  nature  (some  form  of  a  cyclophoria) 
or  of  an  active  nature  (some  form  of  circumduction  or  a  cy- 
cloduction). The  latter  terms  are  in  many  ways  preferable, 
because  we  are  accustomed  to  use  the  termination  "duction  " 
to  describe  other  motions  which  the  globe  makes  in  the  in- 
terest of  single  vision.  The  fact  is,  however,  that  the  terms 
"  extorsion  "  and  "  intorsion  "  have  been  used  so  much  as 
synonymous  with  different  forms  of  cycloduction  that  it 


Hering's  Method. 


257 


seems  better  to  keep  this  nomenclature  until  it  is  changed 
by  the  action  of  national  or  international  ophthalmological 
societies. 

In  order  to  measure  how  far  the  muscles  can  thus  rotate 
the  globe  in  a  wheel  motion,  we  naturally  try  to  devise  some 
arrangement  by  which  the  image  of  a  vertical  line,  for  ex- 
ample, falls  on  one  retina,  and  the  image  of  a  similar  line  not 
quite  vertical  falls  on  the  other  retina,  and  then  observe  to 
what  extent  the  eyes  are  rotated,  in  order  to  fuse  these  lines. 
Such  indeed  was  the  plan  followed  even  by  the  earliest 
students  of  this  question.  Thus  we  have : 

(A)  Hering's  Method  (B  586).  The  arrangement  for 
this  appears  simple  in  theory,  though  its  proper  use 
requires  a  trained  observer.  On  a  blackboard  which  meas- 
ures about  80  by  50  centimeters,  two  pieces  of  white  string 
are  attached  as  seen  in  Figure  182.  Each  of  these  is  made 


FIG.  182. — Hering's  arrangement  for  measuring  the  position  of 
the  vertical  axes. 

taut  by  a  bullet  attached  to  the  lower  end.    The  distance 
between  the  two  strings  is  about  six  centimeters.     A  little  to 


258  Hering's  Method. 

their  right  is  a  vertical  red  band  of  wood  about  two  centi- 
meters in  breadth,  attached  to  the  blackboard  by  an  iron  pin 
passing  through  a  hole  in  the  center  of  the  band  in  such  a 
way  that  the  band  can  be  moved  around  the  pivot  to  one  side 
or  the  other,  revolving  in  the  plane  of  the  blackboard.  To 
the  lower  portion  of  this  band  another  string  is  attached,  one 
end  of  which  passes  horizontally  to  the  right,  through  an 
eyelet  on  the  blackboard,  and  the  other  end  in  the  same 
manner  to  the  left.  The  object  of  this  string  with  its  two 
long  free  ends  is  to  allow  the  observer  to  tip  the  vertical  red 
band  more  conveniently  when  he  is  at  some  little  distance 
from  the  blackboard.  The  upper  end  of  the  vertical  red 
band  is  pointed  and  moves  along  a  graduated  arc. 

To  make  the  measurement,  the  observer  first  steadies  the 
head,  preferably  by  resting  his  teeth  on  the  wooden  bar  of  a 
Helmholtz  bit  in  one  of  its  various  forms  (B  257,  p.  657),  and 
in  doing  so  brings  the  left  eye  about  opposite  an  imaginary 
vertical  line  between  the  weighted  cords.  The  right  eye  is 
opposite  the  vertical  band,  and  both  are  on  a  horizontal 
plane  which  passes  through  the  pivot  in  the  vertical  band. 
If  it  is  desired  to  ascertain  the  position  of  the  vertical  axes 
when  the  visual  axes  are  parallel,  of  course  the  blackboard 
must  be  at  a  distance  of  at  least  five  or  six  meters.  The 
observer  then  looking  steadfastly,  as  in  the  effort  to  obtain 
stereoscopic  vision,  endeavors  to  make  the  band  appear  be- 
tween the  two  vertical  strings.  In  order  to  accomplish  this,  it 
is  often  necessary  to  hold  a  sheet  of  cardboard  or  other  screen 
vertically  in  the  median  line,  having  the  cardboard  so  large 
that  the  right  eye  can  see  only  the  band,  and  the  left  eye 
only  the  vertical  strings.  Usually,  in  doing  so,  the  upper 
end  of  the  vertical  axis  of  each  eye  actually  turns  a  trifle 
outward.  The  observer  then  draws  on  the  horizontal  string 
which  comes  from  the  lower  part  of  the  band  and  tips  the 
upper  end  of  the  band  a  trifle  to  the  left.  The  distance 
which  the  upper  end  of  the  red  band  thus  traverses  in  order 
to  appear  vertical  indicates  the  amount  which  the  upper  end 
of  the  vertical  axis  of  each  eye  has  tipped  outward. 

Hering's  method  was  the  one  used  principally  by  Aubert 
and  Landolt  (B  820)  in  obtaining  the  data  which  will  be  dis- 


Donders's  Method 


259 


cussed  in  connection  with  the  torsion  which  accompanies 
convergence. 

(B)  Donders's  Method. — Another  early  worker  at  this 
problem  was  Bonders  (B  590).  His  apparatus  was  simple, 
but  necessitated  a  little  care  in  having  it  perfectly  leveled. 
The  vertical  lines,  which  in  this  case  are  represented  by  wires, 
are  attached  to  a  frame  which  moves  over  a  second  one.1 

This  apparatus  he  called  an  "  isoscope."  The  principle  in- 
volved is  seen  at  a  glance  (Fig.  183),  and  the  method  of  mak- 
ing these  measurements  is  entirely  similar  to  Hering's. 


u 


FIG.  183. — The  isoscope  of  Donders. 


(C)  Measurements  of  Torsion  with  Parallel  Visual 
Axes  by  means  of  the  Maddox  rod. — This  rod  can  be  made 
to  produce  a  simple  line  on  the  retina,  and  as  the  tests  which 
we  are  now  considering  depend  on  fusion  of  the  images  of 
such  a  line  on  each  retina,  it  has  been  used  by  students  for 
this  purpose.  Duane  (B  558)  refers  to  previous  attempts  of 
the  kind,  and  describes  the  form  which  seemed  to  him  best 

1  On  attempting  to  verify  these  experiments  it  was  found  more  convenient 
to  dispense  with  one  of  the  wires  and  to  supplant  it  by  a  plumb  line  as 
shown  in  the  figure. 


260 


Measurements  with  glass  Rods. 


adapted  to  such  work.  This  consists  briefly  of  two  com- 
pound rods  mounted  on  the  frame  which  had  been  described 
by  Stevens  for  his  phorometer.  The  rod  or  series  of  rods 
before  one  eye  is  made  of  white  glass,  and  that  before  the 
other  of  colored  glass.  When  these  two  rods  are  placed 
horizontally,  each  one  form's  on  the  retina  a  vertical  line. 
The  power  effusion  of  the  retinal  images  shows  what  degree 
of  torsional  power  exists,  the  mechanical  arrangement  of 
Stevens's  phorometer  indicating  with  considerable  exact- 
ness the  amount  of  inclination  which  is  given  to  each  rod 
and  therefore  to  the  retinal  imaees. 


FIG.  184. — Arrangement  by  the  author  of  the  glass  rods  of 
Maddox  for  measuring  torsion  with  parallel  axes. 

I  have  also  found  that  the  same  principle  can  be  used  with 
advantage  by  slightly   modifying  a  part  of  the  apparatus 


Measurements  with  Volkmann's  Discs.        261 

which  is  employed,  as  we  shall  see  later,  to  measure  relative 
accommodation,  Fig.  184.  In  this  a  Maddox  rod  is  placed 
before  each  eye,  there  being  an  index  above  to  show  the  de- 
gree of  inclination  given  to  it.  The  source  of  light  may  be 
placed  at  a  distance,  or  even  approached  within  the  usual 
five-  or  six-meter  limit,  for  as  each  rod  blurs  the  image  of  the 
flame,  efforts  at  convergence  are  eliminated. 

(D)  Modifications  of  Volkmann's  discs  for  the  measure- 
ment of  torsion  with  parallel  visual  axes.  We  have  already 
seen  that  Volkmann's  discs  give  the  most  accurate  meas- 
urement of  cyclophoria  when  one  disc  has  a  radius  ex- 
tending in  one  direction,  and  the  other  a  radius  in  the 
opposite  direction.  In  that  form  it  constitutes  the  essential 
part  of  the  clinoscope  described  by  Stevens.  In  a  similar 
way  such  discs  can  be  used  to  measure  torsion  with 
parallel  axes,  provided  we  draw  on  each  disc  a  complete  di- 
ameter (Fig.  185).  It  is  evident  that  such  discs  can  be  easily 
adjusted  to  any  of  the  clinoscopes  mentioned,  and  serve 
an  excellent  purpose. 


00  0 


A'  8' 


00  0 


r 


FIG.  185.  —  Diameters  and  radii  used  on 
Volkmann's  discs. 

In  order  to  understand  the  difference  between  the  measurements  with  the 
radii  alone,  and  with  the  entire  diameters  when  these  are  turned  at  an  angle 
with  each  other,  let  us  observe  these  discs  of  the  clinoscope  more  carefully. 
In  some  descriptions  of  that  instrument,  each  disc  is  figured  with  a  vertical 
diameter  as  in  A  and  B  (Figure  185)  Now  it  is  known  that  if  the  upper  end 


262  Maximum  and  Minimum  Torsion. 

of  each  of  these  diameters  is  revolved  outward  from  the  median  line  so  that 
the  diameters  come  to  occupy  such  a  position  as  we  see  in  A'  and  B',  then  as 
each  eye  looks  through  one  of  the  tubes  of  the  clinoscope,  the  observer  can 
still  see  one  disc  with  one  vertical  diameter,  as  in  C.  Indeed,  it  makes  but 
little  difference  whether  each  one  of  these  vertical  diameters  is  tipped  out- 
ward or  inward  a  certain  number  of  degrees,  or  whether  one  of  them  remains 
vertical  and  the  other  is  tipped  out  or  in  for  a  correspondingly  greater  num- 
ber of  degrees,  they  can  still  be  fused  into  the  one  vertical  diameter.  This 
experiment  can  be  verified  either  by  the  clinoscope,  or  by  any  similar  arrange- 
ment of  nearly  parallel  lines.  But  the  explanation  which  is  often  given  of  this 
phenomenon  is  erroneous — at  least  in  part.  Thus  it  is  said  that  when  the  upper 
end  of  either  disc  is  tipped  inward,  then,  in  the  interest  of  single  vision,  the 
corresponding  eye  rotates  also.  In  this  way  it  is  supposed  that  the  image  of 
the  line  A,  even  when  it  is  rotated  to  the  position  A',  falls  on  a  part  of  the 
retina  which  "  corresponds  to"  the  upper  part  of  the  image  of  the  diameter  in 
B'  and  for  that  reason — namely,  because  the  images  of  the  upper  parts  of  the 
diameters  in  A'  and  B'  fall  on  parts  of  the  retina  of  the  left  and  right 
eyes  respectively  which  "correspond"  to  each  other, — the  images  of  those 
two  lines  are  fused  in  the  brain  of  the  observer  into  one  vertical  line,  such  as  he 
sees  in  C.  This  explanation  is  not  entirely  true.  The  experiment  does  not  meas- 
ure the  amount  of  real  wheel  motion  of  the  eyes,  if  a  whole  diameter  of  the  disc 
be  used.  This  can  be  easily  shown.  For,  let  us  leave  the  vertical  diameter  A 
as  it  is,  opposite  the  left  eye,  and  before  the  right  one,  instead  of  the  single 
diameter  B,  let  us  place  a  disc  which  has  two  radii  bent  at  an  angle  to  each  other 
as  in  B".  When  the  observer  looks  through  the  two  tubes,  he  can  usually  fuse  the 
line  A  and  the  broken  line  B"  into  one  vertical  line  C.  As  it  is  evidently  impos- 
sible for  the  right  eye  to  turn  in  two  directions  at  the  same  time,  the  only  expla- 
nation of  this  result  is  that  the  upper  and  also  the  lower  end  of  the  broken  line 
B"  can  each  be  made  apparently  to  "correspond"  with  the  upper  and  the  lower 
ends  respectively  of  the  vertical  diameter  in  A,  even  though  these  different 
points  in  the  two  eyes  do  not  actually  correspond  with  each  other  in  any  way. 
In  other  words,  we  must  accept  the  view  of  Verhoeff,  that  what  we  call  "cor- 
responding points  "in  the  retina  is  rather  a  relative  term,  and  that  points 
which  do  not  anatomically  correspond  with  each  other  can  still  be  made  to  do 
so,  within  a  certain  small  range,  before  double  vision  is  produced. 

§  4.  Difference  between  "Maximum"  and  "Minimum" 
Extorsion  or  Intorsion.— If,  in  any  form  of  the  clinoscope, 
a  Volkmann's  disc,  with  an  entire  diameter,  is  placed  verti- 
cally before  one  eye,  and  a  similar  disc  vertically  before  the 
other  eye,  and  the  two  discs  be  fused  into  one,  it  will  be  found 
that  one  or  both  discs  can  be  slowly  rotated  outward  or  in- 
ward five  or  six  degrees  or  more,  while  the  observer  still 
sees  but  one  disc.  In  other  words,  when  the  images  are  first 
fused,  the  instinctive  desire  to  continue  that  fusion  is 
sufficient  to  cause  one  eye  or  both  to  make  a  certain 


Clinical  Importance  of  Torsion.  263 

"  circumduction,"  or  "  cycloduction,"  or  ex-  or  "  in-torsion"  as 
we  prefer  to  call  it.  This  is  the  maximum  extorsion  or  intor- 
sion,  as  the  case  may  be.  On  the  other  hand,  if  the  diameter 
on  the  disc  before  one  eye  is  vertical,  and  the  diameter  be- 
fore the  other  eye  is  turned  outward,  say  eight,  six,  or  even 
four  degrees,  the  same  person  who  before  fused  the  two  di- 
ameters at  this  point  can  not  do  so  until  they  are  brought 
very  much  nearer  to  the  same  position.  In  other  words, 
when  the  retinal  images  are  first  dissociated,  the  muscles 
rotate  the  globe  only  a  comparatively  few  degrees  in  the 
effort  to  fuse  the  two  images.  This  may  be  termed  the 
minimum  power  of  intorsion  or  extorsion.  Evidently  in  any 
examinations  made  with  the  Volkmann  discs,  this  difference 
must  be  kept  in  mind. 

§  5.  The  Physiological  Amount  of  the  Maximum  or 
Minimum  Extorsion  or  Intorsion  of  which  the  eyes  are 
capable  when  the  axes  are  in  the  primary  position  has  appar- 
ently not  yet  been  determined  by  the  examination  of  any 
considerable  number  of  persons.  Among  the  soldiers  and 
students  already  referred  to  there  was  sometimes  inability 
to  fuse  the  discs  at  all,  but  in  twenty-three  non-asthenopic 
persons  the  tests  were  sufficiently  accurate  to  be  reliable. 
Among  these  it  was  found  that  with  the  eyes  in  the  primary 
position  there  was  an  average  minimum  intorsion  of  about 
two  and  a  half  degrees  and  minimum  extorsion  of  about  four 
degrees.  The  number  of  persons  is  so  small,  however,  as  to 
serve  only  as  an  indication  of  what  may  be  established. 
Maximum  ex-  and  intorsion  are  often  more  than  twice  as 
much  as  the  minimum,  but  vary  greatly  in  different  persons. 

§  6.  What  Evidence  is  there  that  this  Form  of  Torsion 
is  of  any  Clinical  Importance  ? — Many  ophthalmologists  are 
inclined  to  consider  this  rather  a  question  of  laboratory  in- 
terest, or  the  fad  of  a  few  enthusiasts.  Indeed,  the  study  of 
torsion  in  any  form  has  been  much  neglected  because  of  ex- 
travagant claims  concerning  its  importance  and  methods  of 
treatment.  As  a  result,  the  average  practitioner  is  apt 
to  class  all  together  and  consign  them  to  oblivion.  But 
a  simple  experiment  with  the  Volkmann  discs  is  sufficient 
to  show  the  pathological  effect  of  even  small  variations  in  the 


264  Clinical  Importance  of  Torsion. 

amount  of  torsion.  Thus,  when  with  parallel  axes  the  mini- 
mum  intorsion  is  about  three  degrees,  if  the  discs  be  adjusted 
to  a  slightly  greater  number  of  degrees  and  kept  there  even  for 
a  few  minutes,  the  sense  of  discomfort  is  very  marked  and  by 
persistence  becomes  extremely  annoying.  But  this  is  nothing 
more  than  the  condition,  in  some  cases  at  least,  in  certain 
forms  of  astigmatism  where  the  axes  are  sufficiently  near  to 
each  other  to  permit  a  constant  effort  on  the  part  of  the  in- 
dividual to  fuse  the  retinal  images.  If  such  a  person  focuses 
vertical  lines  clearly  with  one  eye,  and  if  with  the  other  eye, 
when  making  the  same  effort  at  accommodation,  he  naturally 
focuses  lines  which  are  slightly  oblique,  we  would  expect  him 
to  make  the  same  effort  at  torsion  as  occurs  when  such  lines  are 
viewed  on  the  Volkmann  discs.  In  fact,  that  is  just  what  we 
do  find  not  infrequently.  The  evidence  is  abundant  that 
when  the  axes  of  astigmatism  approach  each  other,  but  are 
still  sufficiently  divergent  to  produce  this  effort  at  torsion, 
that  condition  is  a  very  important  cause  of  asthenopic 
symptoms,  even  though  the  degree  of  astigmatism  be  slight. 
This  phase  of  the  subject  will  be  elaborated  in  a  sub- 
sequent chapter. 


CHAPTER  VI. 

SECOND   GROUP  OF  ASSOCIATED   MOVEMENTS. 

Definition.  The  parallel  visual  axes  move  in  one  of  the 
principal  meridians  to  the  right,  left,  up,  or  down.  In  these 
movements  there  is  no  torsion.  This  is  shown  by  very  simple 
experiments  with  after-images,  according  to  the  plan  which 
was  first  suggested  by  Ruete.  They  are  more  easily  fol- 
lowed if  we  confine  our  attention  to  the  vertical  axis  of  one 
eye  only. 

If  we  wish  to  ascertain  what  position  the  after-images 
assume  when  one  eye  moves  (and  the  other  one  moves  with 
it)  in  any  one  of  the  principal  meridians,  we  do  not  require 
to  have  before  us  any  surface  mapped  off  exactly.  One  of 
the  simplest  methods  of  obtaining  a  vivid  after-image,  and 
one  which  serves  our  purpose  in  this  case,  is  to  allow  the  sun- 
light to  enter  a  dark  room  through  a  narrow  slit. 

Let  us  suppose  that  we  wish  to  ascertain  what  changes  ap- 
pear in  the  after-image  of  a  vertical  slit  when  parallel  visual 
axes  move  in  the  principal  meridians.  The  observer  sits 
opposite  a  vortical  slit  in  a  shutter,  and  if  the  sun  is 
shining  brightly,  in  a  few  seconds  the  .image  of  the  slit  is 
branded  on  the  retina.  Now  if  the  eye  be  turned  straight 
up  and  down,  the  after-image  of  the  slit  remains  vertical.  It 
often  happens,  in  making  this  experiment,  that  as  the  eye 
moves  up  and  down,  for  example,  the  vertical  after-image 
seems  to  b-e  vertical,  but  moves  somewhat  obliquely  up  and 
down.  This  is  because  the  head  is  not  held  perfectly 
straight,  and  by  tipping  it  a  little  to  one  side  or  to  the  other 
it  is  easy  to  see  that  there  is  no  change  in  the  direction  of 
the  vertical  axes  as  the  eyes  make  this  movement.  If  the 
vertical  line  in  the  shutter  is  changed  to  one  which  is  hori- 
zontal, movements  of  the  eyes  from  side  to  side  give  similar 

265 


266     Second  Group  of  Associated  Movements. 

results.  We  establish  for  ourselves  in  this  way  the  fact  that 
in  these  associated  movements  of  the  second  group  the  verti- 
cal axes  remain  practically  vertical,  and  the  horizontal  axes 
remain  horizontal.  The  clinical  bearing  of  this  fact  is  evi- 
dent in  connection  with  the  position  of  the  double  images 
which  accompany  paralyses. 


CHAPTER   VII. 

THIRD   GROUP   OF  ASSOCIATED   MOVEMENTS. 

§  i.  Definition. —  The  parallel  visual  axes  move  obliquely. 
In  this  group  the  vertical  and  horizontal  axes  apparently  change 
their  positions  in  such  a  manner  as  to  produce  "false  torsion'' 
This  movement  from  a  primary  to  a  secondary  position  is 
illustrated  in  Figure  186.  Thus,  if  the  anterior  end  of  the 


FIG.  186. — Arc  described  when  the  visual  axis  A  B  passes  from  the 
primary  into  some  secondary  position — for  example,  to  E  (Meissner). 

visual  axis  passes  from  the  point  B,  in  the  primary  posi- 
tion, 'to  the  point  E,  the  changes  produced  are  the  same 
as  though  the  eye  reached  that  position  first,  by  passing  to 
the  point  K  and  then  upward  to  E>  or  from  B  to  H  to 

267 


268      Third  Group  of  Associated  Movements. 


E,  or  through  any  other  point,  no  matter  where  it  is  situ- 
ated, to  the  point  E. 

Now,  in  any  movement  of  this  group,  what  we  call  the 
vertical  axis  at  one  instant,  ceases  to  be  the  vertical  axis  as 
soon  as  the  globe  has  changed  its  position.  The  result  is 
an  apparent  rolling  of  the  globe,  though  not  a  true  wheel 
motion.  With  proper  precautions,  this  can  be  seen  in  the 
movement  of  a  given  spot  in  the  iris. 

This  movement  was  first  described  simply  as  "  torsion  " 
or  "Rollungen"  by  Helmholtz  (B  584)  and  by  Bonders 
(B  590),  while  Maddox  (B  263)  and  some  of  the  other 
English  writers  properly  call  it  "  false  torsion,"  to  distin- 
guish it  from  the  true  wheel  motion.  Objectively,  or  as  far 
as  the  motion  is  concerned,  false  torsion  is  a  twisting 
motion,  if  it  might  be  so  called,  about  the  antero-posterior 
diameter,  though  of  course  there  is  no  twisting  of  the  globe 
as  a  rope  is  twisted.  Subjectively,  it  is  the  distortion  which 


•4- H h 


FIG.    187. — Position   which   the   after-images   assume 
when  projected  on  a  flat  background  (Listing). 

an  object  seems  to  undergo  when  that  object  is  viewed  on 
a.  flat  surface,  like  a  wall. 

The   facts  with   regard   to    false   torsion  can  be  studied 


After-images.  269 

best  by  means  of  the  after-images.  If  we  wish  to  determine 
accurately  the  amount  of  tipping  which  after-images  undergo, 
it  is  necessary  to  have  a  suitable  background  as  a  measure 


FIG.  188. — Curves  of  which  the  after-images  form  a  part. 

upon  which  those  images  can  be  projected.  Such  a  back- 
ground cannot  be  easily  distinguished  in  a  darkened  room, 
and  we  must  therefore  modify  the  plan  followed  when 
studying  the  last  group  of  associated  movements.  On  a 
well-illuminated  white  wall  about  one  meter  square,  or,  still 
better,  on  a  piece  of  enameled  cloth,  we  draw  cross  lines  in 
black  such  as  are  represented  in  Figure  187.  At  the  center 
we  attach  a  cross  (A)  of  bright  red  ribbon,  about  ten  centi- 
meters long  and  one  centimeter  broad.  With  this  white 
surface  well  illuminated  and  the  center  of  the  cross  about  on 
a  level  with  the  eyes,  and  with  accommodation  well  relaxed, 
let  the  chin  of  the  observer  rest  on  a  suitable  support,  so 
that  the  head  is  easily  retained  in  the  same  position.  Then, 
after  looking  intently  at  the  central  cross  long  enough  to 
brand  its  image  upon  the  retina,  it  is  well  first  to  turn  the 


270  After-images. 

eye  straight  up  or  down,  and  the  after-image  should  slide 
along  the  vertical  meridian  on  which  the  red  band  is  drawn. 
Or  if  not,  it  can  be  made  to  do  so  by  tipping  the  head 
slightly,  or  turning  it  from  side  to  side  in  order  to  bring  the 
head  into  just  the  vertical  position. 

After  having  thus  adjusted  the  position  of  the  head  with 
reference  to  the  vertical,  and  in  a  similar  manner  to  the  hori- 
zontal planes,  the  visual  axes  can  be  directed  obliquely. 
When  that  is  done,  however,  it  is  noticed  at  once  that,  as 
the  after-image  is  projected  on  the  flat  wall  in  front,  the  cross 
no  longer  remains  a  perfect  cross,  but  seems  to  be  distorted, 
and  this  distortion  increases  in  proportion  as  the  point  looked 
at,  is  distant  from  either  the  horizontal  or  the  vertical  me- 
ridian. Indeed,  with  a  little  practice  it  is  easy  to  see  that 
the  increase  in  the  amount  of  distortion  is  dependent  upon 
two  factors.  One  of  these  is  the  distance  of  the  point  looked 
at  from  the  horizontal  meridian,  and  the  other  is  its  distance 
from  the  vertical  meridian.  It  is  well  to  note  this  fact,  be- 
cause it  forms  the  basis  of  all  the  calculations  concerning 
false  torsion.  The  position  which  the  arms  of  the  cross  as- 
sume is  seen  in  Figure  187.  It  is  not  easy  to  keep  in  mind 
what  these  positions  are,  but  they  will  be  recalled  easily  if 
we  remember  that  the  distorted  cross  would  form  part  of  the 
curve  if  the  vertical  lines  curved  inward  toward  the  central 
cross,  and  the  horizontal  lines  also  curved  toward  the  cross, 
as  seen  in  Figure  188. 

The  foregoing  changes  in  the  position  of  the  after-images 
when  the  axis  is  in  an  oblique  position  are  those  seen,  as  we 
must  remember,  when  the  images  are  projected  on  a  flat 
surface.  It  is  these  images  which  were  studied  by  the  earliest 
writers,  and  from  them  most  of  the  conclusions  were  drawn 
concerning  the  corresponding  changes  which  the  eye  also  was 
supposed  to  undergo  when  assuming  an  oblique  position. 
But  the  important  fact  is  that  the  position  of  the  lines  of  the 
cross  are  distorted  only  because  they  are  projected  upon 
the  flat  surface. 

Tscherning  (B  645)  made  a  complete  and  beautiful  demon- 
stration of  this  fact.  He  constructed  a  hemisphere  of  con- 
siderable size,  and  placing  the  head  at  its  center,  he  looked 


Nature  of  False  Torsion. 


271 


at  the  cross  immediately  in  front  until  its  image  was  impressed 
on  the  retina,  and  then  turning  the  eyes  in  various  oblique  di- 
rections, he  found  the  after-images  were  not  exactly  the 
same  as  the  after-images  projected  on  a  flat  wall,  but  ap- 
peared as  in  Fig.  189. 


FIG.  189. — Position  which  the  after-image  assumes  when  the  cross  is 
projected  on  the  concave  surface  of  a  hollow  hemisphere  (Tscherning). 

§  2.  This  Form  of  Torsion  is  not  a  True  Wheel  Motion. 

• — In  order  to  obtain  a  clearer  idea  of  the  motions  of  this  group, 
let  us  make  use  of  an  ophthalmotrope  in  some  one  of  its  forms. 
Take,  for  example,  the  simple  rubber  ball  transfixed  by  three 
needles  and  add  to  it  as  follows : 

1st.  Attach  a  fine  thread  to  the  anterior  polar  axis  as  it 
emerges  from  the  globe.  Make  this  thread  a  little  longer 
than  the  radius  of  the  circle  which  represents  the  cornea, 
and  to  the  loose  end  of  the  thread  attach  a  small  bead,  a  bit 
of  shoemaker's  wax,  or  some  other  object  heavy  enough  to 
make  a  plumb  line  of  it. 

2d.  Mark  off  about  thirty  or  forty  degrees  at  the  lower 
edge  of  the  circle  which  indicates  the  edge  of  the  cornea,  or 
any  other  circle  on  the  eye  concentric  with  it. 

3d.  Transfix  the  rubber  ball  with  still  another  knitting 
needle,  which  shall  constitute  the  axis  upon  which  the  globe 
revolves  into  the  oblique  position  to  which  it  is  to  turn. 

If  we  prefer  to  use  for  this  purpose  the  more  complete 


272 


Nature  of  False  Torsion. 


ophthalmotrope,  it  is  only  necessary  to  attach  to  the  front 
part  of  the  globe  a  projecting  point  which  represents  the  an- 
terior end  of  the  polar  axis  ;  the  graduation  near  the  edge  of 
the  cornea  is  already  marked,  and  by  changing  the  position 
of  the  radial  pins  we  allow  the  globe  to  turn  into  almost  any 
position  desired. 

When  the  visual  axis  passes  up  and  outward,  for  instance, 
the  plumb  line  shows  that  what  was  the  vertical  axis  in  the 
primary  position  no  longer  remains  so,  but  that  another  axis 
which  marks  another  vertical  plane  has  taken  its  place.  The 
number  of  degrees  between  these  two  plumb  lines  marks 
evidently  the  amount  of.  or  the  angle  of  false  torsion.  But 
during  this  act  the  globe  has  not  really  revolved  directly  upon 
the  visual  axis.  In  other  words,  this  form  of  so-called  torsion 
is  only  a  result  of  two  motions  which  the  globe  has  made  out 
and  upward.  Later  we  will  consider  more  exactly  how  the 
exact  amount  of  this  torsion  can  be  calculated,  but  at  present 
let  us  continue  with  the  question  immediately  before  us — 
namely,  the  nature  of  the  motion  itself. 

This  can  also  be  illustrated  very  readily  by  a  simple  cir- 
cular disc  of  cardboard,  as  suggested  by  Le  Conte  (B  825,  p. 
198)  and  elaborated  by  Maddox  (B  263,  p.  47). 
The  former  says  :     "A  simple  experiment  will  show  the 

kind  of  rotation  which 
takes  place  in  bringing 
the  eye  to  an  oblique  posi- 
tion. Take  a  circular  card 
(Fig.  190)  and  make  on  it 
a  rectangular  cross,  which 
shall  represent  the  vertical 
(VV)  and  horizontal  (H  H) 
meridians  of  the  retina. 
A  small  circle/  represents 
the  pupil.  Now  take  hold 
of  the  disc  with  the  thumb 
FIG.  190.— Cardboard  model  to  illustrate  and  finger  of  the  right  hand 
false  torsion.  at  the  points  VV,  and  place 

this  line  in  a  vertical  plane.  Then  tip  the  disc  up  so  that 
the  pupil/  shall  look  upward  45°  or  more,  but  the  line  VV 


Nature  of  False  Torsion.  273 

still  remaining  in  the  vertical  plane.  Finally,  with  the 
finger  of  the  left  hand,  turn  the  disc  on  the  axis  VV  to  the 
left.  It  will  be  seen  that  VV  is  no  longer  vertical,  nor  HH 
horizontal,  but,  some  other  line — XX — is  vertical  and  YY 
horizontal.  In  other  words,  the  whole  disc  seems  to  have 
rotated  to  the  left.  But  there  is  evidently  no  true  rotation 
on  a  polar  axis,  but  only  an  apparent  rotation  consequent 
upon  reference  to  a  new  vertical  meridian  of  space." 

There  is  often  so  much  confusion  concerning  this  point 
that  it  is  worth  while  thus  to  vary  the  illustration,  even  at 
the  risk  of  tedious  repetitions. 

§  3.  Conclusions  Regarding  the  Data  Obtained  by 
these  Experiments  with  the  After-images.— At  this  point 
it  is  desirable  to  review  the  data  which  we  have  collected 
from  these  experiments  and  to  summarize  in  as  few  words  as 
possible  the  conclusions  to  which  we  have  been  led.  We 
can  then  see  how  these  conclusions  of  our  own  compare 
with  the  more  condensed  statements  concerning  the  same 
motions  which  have  been  given  by  others,  and  which  have 
come  down  to  us  in  the  literature  as  the  laws  of  Donders 
and  Listing.  We  have  found  : 

1st.  When  the  eye  moves  from  the  primaiy  position,  up, 
down,  right,  or  left,  as  in  movements  of  the  second  group, 
no  torsion  occurs. 

2d.  When  the  eyemoves  into  an  oblique  position  the  axis 
which  is  vertical  in  the  primary  position  is  supplanted  by 
another  vertical  axis,  the  former  vertical  axis  being  then 
oblique. 

3d.  The  after-images  which  we  see  projected  on  a  fla: 
surface  are  not  in  the  same  position  when  projected  on  a 
concave  surface  to  which  the  visual  axis  is  perpendicular. 

4th.  This  so-called  false  torsion  is  not  a  true  wheel  motion 
of  the  globe. 

§  4.  Bonders'  s  Law. — With  these  conclusions  before  us 
it  is  easier  to  understand  the  more  exact  "  laws  "  or  state- 
ments concerning  these  movements  which  have  been  for- 
mulated by  Donders  and  Listing. 

First,   as   to   Donders' s  law.     This  is  usually  stated  by 
saying  that  "  the  wheel  motion  of  each  eye  with  parallel 
18 


274  Listing's  Law. 

fixation  lines  is  a  function  only  of  an  elevation  and  of  a 
lateral  deflection."  Donders'  s  law  is  therefore  only  another 
way  of  stating  what  we  have  seen  with  the  after-images 
of  the  cross  on  the  squares — namely,  that  the  amount  of 
this  false  torsion  depends  on  two  factors, — the  number 
of  degrees  which  the  visual  axis  turns  up  or  down,  and  the 
number  of  degrees  which  that  axis  turns  in  or  out. 

§  5.  Listing's  Law. — According  to  this,  "  when  the  line 
of  fixation  passes  from  its  primary  to  any  other  position,  the 
angle  of  torsion  of  the  eye  in  this  second  position  is 
the  same  as  if  the  eye  had  arrived  at  that  position  by 
turning  about  a  fixed  axis  perpendicular  to  the  first  and 
second  positions  of  the  line  of  fixation." 

The  fact  is  that  Listing  did  not  formulate  that  law  at  all. 
He  gave  the  principle,  and  later,  in  1853,  Ruete  gave  the 
words, — unfortunately,  obscure  ones.  For  as  Mauthner  ob- 
served, "  simple  as  this  law  is,  and  as  it  sounds,  certainly  no 
one  ever  understood  it  at  first  hearing."  It  is  doubtless  a 
source  of  consolation  to  most  of  us  to  know  that  a  man  long 
trained  in  problems  of  physiological  optics  also  finds  this 
condensed  statement  difficult  to  understand.  Yet  Listing's 
law  only  expresses  in  another  way  what  we  have  seen 
already  in  these  experiments  with  after-images  of  the  cross. 
Concerning  this  subject,  Tscherning  says:  "  The  law  of 
Donders  may  be  considered  as  undergoing  further  develop 
ment  in  the  law  of  Listing.  The  former  indicates  how  the 
position  of  the  after-image  is  determined  by  a  given  position 
of  the  visual  axes;  the  latter  tells  us  what  that  posi- 
tion is." 

§  6.  Calculation  of  the  Amount  of  Torsion  with  Parallel 

Axes. — Having  thus  seen  what  torsion  with  parallel  visual 
axes  is,  how  it  is  produced,  and  how  it  can  be  illustrated 
objectively,  we  will  follow  it  one  step  further  to  ascertain  its 
exact  extent  with  any  given  position  of  the  visual  axis. 
This  was  calculated  by  Helmholtz  (B  257,  p  624)  and  most  of 
the  following  table  was  published  in  his  Physiological  Optics. 
His  table,  however,  was  not  carried  quite  far  enough  to  show 
the  torsion  which  occurs  with  certain  movements  up  or 


Bonders' s  Law  275 

down,  which  are  quite  common.     It  has  therefore  been  ex- 
tended and  in  the  elaborated  form  is  as  follows 


In 
or 
out 

5° 

10° 

15° 

20° 

Up  or  down. 
25°     30° 

35° 

40° 

45° 

5U 

o°  13' 

o°  26' 

o°  40' 

o°53 

1-07' 

i  "  20' 

i°  35' 

,  i° 

49' 

2° 

04' 

10° 

o°  26' 

o°53* 

i°  19' 

i°46' 

2°  13' 

2°  41' 

3°  10' 

3° 

39' 

4° 

09' 

15° 

o°  40' 

i°  19' 

i°59' 

2°  40' 

3°  21' 

4  "02' 

4°  45' 

5° 

29' 

6° 

15' 

20° 

o°  53' 

i°46' 

2°  40' 

3°  34' 

4°  29' 

5°  25' 

6°  22' 

7° 

21' 

8° 

2l' 

25° 

I°07' 

2°  13' 

3°  21' 

4°  29' 

5°  33' 

6°  48' 

8°  o' 

9° 

14' 

10° 

30' 

30° 

1°  2l' 

2°  41' 

4°  02' 

5°  25' 

6°  48' 

8°  13' 

9°39' 

11° 

08' 

12° 

40' 

35° 

i°35' 

3°  10' 

4°  45' 

6°  22' 

8°  o' 

9°  39' 

II°2l' 

13° 

06' 

14° 

53 

40° 

i°49' 

3°  39' 

5°  29' 

7°2l' 

9°  14' 

n°  08' 

I3°o6' 

15° 

05' 

i?" 

09' 

§  7.  Aids  to  the  Calculation  of  Torsion  with  Parallel 
Visual  Axes. 

As  this  calculation  is  not  a  simple  one,  and  as  some  student  of  mathematical 
turn  may  care  to  direct  his  energies  toward  the  solution  of  the  problem,  one  or 
two  hints  may  prove  acceptable.  The  best-known  formula  is  the  one  given  by 
Helmholtz  (Physiologische  Optik  second  edition).  In  this  a  is  the  ver- 
tical movement,  ft  the  lateral  movement,  and  y  is  the  size  of  the  angle  of 
rotation. 

sin  a  sin  /? 

—  tan  y  = '— - 

cos  a  +  cos  p 

From  which,  as  Helmholtz  says,  "  there  follows" 
—  tan(v)  =  tang  («)  tang  (|) 

Most  students  will  not  be  able  to  see  at  a  glance  how  this  second  equation 
"follows"  from  the  first  one,  and  the  intermediate  steps  are  therefore  given. 

In  the  formula  understanding  that  A  is  equal  to  Alpha,  B  is  equal  to  Beta,  G 
is  equal  to  Gamma,  then 

sin  A  sin  B  ,  .-,  ,   A  .       ,  -r, 

tan  G  =  • — — -  o  tan  i  G  =  tan  A  A  tan  £  B 

cos  A  +  cos  B 

cos2  G  = r-^- (2)  derived  from  trig.  form,  sec2  G  =  tan*  G+  * 

tan*  G  -f-  i  v 

sin2  A  sin2  B  ,  . 

tan2  G  = — „-; — : j c— i JTT-  .     Substituting  in  (2) 

cos2  A  -|-  2  cos  A  cos  B  +-cos2B 


cos*  G  =. 


sin2  A  sin2  B  -f-  cos2  A  +  2  cos  A  cos  B+cos2  B 

cos'2  A  -f-  2  cos  A  cos  B  -f-  cos2  B 
cos2  A  +  2  cos  A  cos  B  -f-  cos2  B 
~~sin2  A  sin2  B  -J-  cos2  A  +  2  cos  A  cosB  +  cos2B   . 

Substituting  for  sin*  A  and  sin2  B  their  equals.  I — cos2  A  and  I — cos4  B 

__  (cos  A  -f  cos  B)2 

~~  I— cos*  A — cos2  B  +  cos"  A  cos1  B  +  cos*  A  -f-  cos*  B  -f-  2  cos  A  cos  B 


276  Bonders' s  Law 

(cos  A  -j-  cos  B)2 

~~  i  +  2  cos  A  cos  B  -f-  cos2  A  cos2  B 
„  (cos  A  +  cos  B)2 

COS'   Lr  = fy.  a 

(I  -f-  cos  A  cos  B)2 


cos  G  = 


cos  A  +  cos  B 


i  -|-  cos  A  cos,  B 

I  -4-  cos  A  cos  B — cos  A — cos  B 

i  —  cos  G  =  — — —  — B • 

i  -j-  cos  A  cos  B 

i  -4-  cos  A  cos  B  -4-  cos  A  -f-  cos  B 

I  +  cos  G  =— --j- jl-  -- — ! 

I  -\-  cos  A  cos  B 


FIG.  191. — Anterior  view  of  the  globe  showing  the  position  which 
the  vertical  arm  of  the  cross  assumes  when  the  eyes  pass  from  the 
primary  to  a  secondary  position. 

I  —  cos  G_i  -f- cos  A  cos  B — cos  A  —  cos  B_(i  — cos  A)(r  —  cos  B) 
i  -)-  cos  G~~  i  -\-  cos  A  cos  B  +  cos  A  -}-  cos  B     (i-|-cos  A,  (i-j-cos  B) 
tan2  I  G  =  tan5  ^  A  tan2  £  B 
tan  ^  G  =  tan  i  A  tan  \  B 

Another  and  quite  different  method  of  calculating  the  amount  of  torsion  with 
parallel  visual  axes  has  been  suggested  to  me  by  Mr.  W.  C.  Kralhwohl  and  is 
as  follows: 

If  we  were  to  take  any  point  on  the  cornea,  Fig.  191,  and  let  it  move  in 
every  possible  direction,  it  would  describe  a  sphere  ABEFD.  Let  BAF  be  an 


Donders's  Law  277 

anterior-posterior  vertical  meridian;  also,  let  us  suppose  that  a  cross  is  marked 
upon  the  eye,  and  that  it  moves  from  A  to  C.  If  the  eye  were  to  move  so  that 
there  were  no  torsion,  the  vertical  arm  of  the  cross  would  lie  on  the  meridian 
BC,  but  since  the  eye  moves  so  that  the  cross  keeps  parallel  to  its  former 
position,  it  assumes  the  position  HG,  and  the-  angle  HCB  equals  the  angle 
of  torsion. 

Angle  HCB  =  angle  HCA  —  angle  BCA 
But  since  HC  is  parallel  to  BA,  and   1C  is  a  continuation   of  AC,  therefore 

angle  HCB  =  angle  BAI  —  angle  BCA 

If  we  let  the  angle  of  inclination  of  the    axis  of  movement  to  the  meridian 
DAE  be  denoted  by  x,  and  the  degree  of  rotation  by  y,  then 

angle  CAE  =  x  =  DAI  and  angle  BAI  =  90°  +  x 
Therefore, 

angle  HCB  =  90° +  x — angle  BCA 

In  order  to  find  angle  BCA  we  must  solve  the  triangle  BAG,  in  which  BA 
=  90°,  AC  =  y,  and  angle  BAC  —  90°  —  x 

In  order  to  solve  this  triangle  we  must  first  solve  its  polar  triangle  B'  C'  A', 
Fig.  192,  in  which 


FIG.  192. — Triangle  for  the  calculation 
of  the  position  of  the  images  in  false 
torsion. 

angle  C'  =  180°  —  AB  =  90° 

angle  B'  =  180°  —  y, 

B'  C  =  180°— (90°— x)  =  90°  +  x 

tan  B  C' 

Cos    anele    B  =   -     ,  ,„, 
tan  A  B 

.,„,  tan  B1  C 

I  angent  A  B  = — -. — =rr 

cos  angle  B 

Substituting  for  A'  B',  B'  C  ,  and  angle  B'  their  respective  equals 
1 80°  —  BCA,  90°  +  x,  and  1 80°  —  y 

tangent  (180°—  BCA)  =   a"  ,  °  „         . 
cos  ( i  So  — y) 

.  „„  .       — cot  x  _  cot  x 

—  tangent  BCA  = =  • — 

—  cos  y        cos  y 


278  Importance  of  False  Torsion 

But  in  Figure  191,  since 

angle  HCB  =  90°  -j-  x  —  angle  BCA 

therefore 

angle  BCA  =  90°  +x  — angle  HCB 

—  tangent  BCA  =  —  tan  (50°  +  x  —  angle  HCB) 
cot  x 


=  cot  (x  —  angle  HCB)  = 


cos  y 


tan    (x  —  angle  HCB)  (tan  x)  (cos  y) 

tan  (x  —angle  HCB)  =  (tan  x)  (cos  y) 

§  8.  The  Clinical  Importance  of  False  Torsion. — Why 

is  it  advisable  to  devote  so  much  space  to  the  consideration 
of  torsion  in  this  form,  or  indeed  in  any  form  ? 

First.  It  is  to  clear  up,  if  possible,  some  of  the  obscurity 
which  shrouds  this  point  in  the  minds  of  most  students 
and  it  is  hoped  that  the  attempt  has  been  at  least  partially 
successful. 

Second.  The  relation  of  this  form  of  torsion  to  the  posi- 
tions of  the  double  images  seen  in  certain  cases  of  paralysis 
is  sometimes  important. 

It  is  easy  to  understand  how  any  imperfect  movements  of 
the  globe  will  not  only  distort  but  may  displace  the  images 
of  distant  objects,  and  a  clearer  understanding  of  the  normal 
changes  helps  us  to  comprehend  those  which  are  abnormal. 
At  present  only  a  single  example  of  this  need  be  given. 

Let  us  select  one  of  the  simplest  types — that  of  a  paralysis 
of  the  sixth  nerve  on  the  right  side.  In  that  case  the  axis 
of  vision  of  the  right  eye  turns  toward  the  left  side  and 
we  have  homonymous  double  images  toward  the  right. 
The  image  with  the  left  eye  is  of  course  in  the  normal  posi- 
tion. If  the  patient  be  directed  to  look  at  a  vertical  object 
in  front — a  candle,  for  example, — the  image  of  that  candle 
which  is  seen  with  the  right  eye,  like  the  arm  of  the  cross 
used  for  experimenting  with  after-images,  remains  vertical, 
just  as  does  the  after-image  when  the  visual  axis  passes 
straight  up  or  down.  In  other  words,  in  these  principal 
meridians  there  is  no  torsion. 

But  when  the  candle  is  moved  upward  and  to  the  right,  the 
image  which  is  seen  with  the  right  eye  is  no  longer  vertical, 
the  upper  end  being  tipped  more  or  less  away  from  the 


Importance  of  False  Torsion  279 

median  plane.  The  tendency  of  the  eye  is  toward  the 
same  kind  of  torsion  which  it  undergoes  when  not  para- 
lyzed. When,  however,  in  the  latter  condition  it  acts  more 
or  less  independently  of  the  other  eye,  then  the  tipping 
which  the  corresponding  image  of  the  candle  undergoes 
when  in  this  oblique  position  depends  evidently  upon  three 
factors.  The  first  two  are  stated  in  Bonders' s  law,  and  worked 
out  exactly  in  the  table  by  Helmholtz,  being  the  amount 
which  the  eye  turns  upward  and  then  out,  or  outward  and 
then  up,  as  the  case  may  be.  The  third  factor  is  the  degree 
of  paralysis  which  is  present  in  the  individual  case.  This, 
with  the  other  two,  determines  the  amount  of  inclination 
given  to  the  false  image  of  the  candle. 


CHAPTER   VIII. 

THE   FOURTH    GROUP    OF   ASSOCIATED    MOVEMENTS. 
CONVERGENCE. 

Definition. —  The  visual  axes  do  not  remain  parallel  ut 
converge  toward  each  other.  In  doing  so,  the  upper  end  of 
each  vertical  axis  rotates  slightly  outward,  producing  true 
torsion. 

This  group  of  motions  is  evidently  the  most  important  with 
which  we  have  to  deal,  because  convergence  is  accompanied 
normally  by  a  certain  amount  of  accommodation  and  also  of 
true  torsion.  In  order  to  take  each  step  securely  in  this  part 
of  our  study,  it  is  desirable  to  make  a  digression  here,  that 
we  may  review  briefly  our  data  concerning  prisms  and  the 
more  fundamental  questions  relating  to  convergence.  After 
that  we  can  pass  to  the  relation  of  convergence  either  to 
accommodation  or  to  torsion. 

DIVISION  I. 
Ophthalmologicai  Prisms. 

§  i.  Definition. — An  ophthalmological  prism  is  a  wedge  of 
glass  having  at  least  two  polished  surfaces  which  meet  at  an 
angle.  This  is  called  the  angle  of  refraction.  The  optical 
properties  of  a  prism  are  illustrated  in  Figure  193.  The 
principle  involved  is  the  elementary  one  in  optics,  that  when 
a  ray  of  light  passes  from  one  medium  into  another  which  is 
more  dense,  the  ray  is  bent  toward  a  plane  perpendicular  to 
the  surface  of  the  denser  medium.  The  reverse,  of  course, 
obtains  if  the  ray  passes  from  a  medium  which  is  dense  to 
one  more  rare. 

When  a  prism  is  placed  with  its  angle  of  refraction  toward 
the  nedian  line,  it  is  called  an  adductive  prism.  In  this  posi- 

280 


Numbering  of  Prisms  281 

tion  it  is  also  known  as  a  minus  prism,  for  if  extended  to 
meet  a  corresponding  prism  before  the  other  eye,  the 
two  together  would  have  in  their  refraction  an  effect  analo- 
gous to  that  of  a  concave  or  minus  glass.  When,  however, 
the  angle  is  toward  the  temple,  it  is  called  an  abductive  or  a 
plus  prism. 

§  2.  Numbering  of  Prisms. — As  different  methods  of 
numbering  prisms  have  been  in  vogue  at  different  times,  it  is 
necessary  to  understand  what  is  referred  to  when  they  are 
mentioned.  They  have  been  numbered  : 

(A)  According  to  the  size  of  the  refracting  angle.  This  is 
a  convenient  method,  but  evidently  inaccurate,  as  a  prism 

A1 


FIG.  193. — The  ophthalmological  prism  and  the  displacement  of  the  image 
which  it  produces  (Hansel). 

made  from  glass  which  has  a  high  index  of  refraction  must 
differ  materially  from  another  made  from  glass  of  a  lower 
index.  This  method  therefore  has  been  to  a  great  extent 
discarded,  though  still  employed  by  some  European  manu- 
facturers. 

(B)  A  much  more  exact  method  was  proposed  by  Dennett 
(B  709),  according  to  what  he  termed  the  "centrad,"  that 
being  a  deviation  whose  arc  is  one  one-hundreth  (-j-J-g-)  of  the 
radius.     Although  perfectly  exact  and  entirely  in  accord  with 
similar  mathematical  calculations,  this  method  of  numbering 
prisms  has  not  been  very  generally  adopted. 

(C)  Another  method  of  numbering  is  according  to  the  so- 
called  "prism-diopter"  of  Prentice  (B  712).     That  is,  accord- 
ing to  the  amount  of  displacement  which  a  prism  would 
produce  when  held  at  a  distance  of  one  meter  from  a  given 
object.     Thus  if  a  meter  measure  is  placed  horizontally  on 


282      Fourth  Group  of  Associated  Movements 


the  wall,  and  the  observer,  standing  opposite  its  right-hand 
end  and  at  a  distance  of  one  meter,  looks  at  that  end  of  the 
measure  through  a  prism  with  its  refracting  angle  toward  his 
left  and  in  the  position  of  minimum  deviation,  then,  if  the 
displacement  produced  amounts  to  three  centimeters,  the 
prism  would  be  called  No.  3.  If  it  produced  a  displacement 
of  five  centimeters  it  would  be  called  prism  No.  5,  and  so  on. 

In  reality,  a  prism  whose  refracting  angle  measures  3  de- 
grees with  the  goniometer,  does  not  produce  a  minimum 
deflection  of  exactly  3  centimeters  when  placed  at  a  distance 
of  one  meter  from  the  object.  The  difference,  however,  is 
very  slight,  and  we  are  evidently  safe  in  adopting  the  "  prism- 
diopter  "  method  of  numbering  prisms  as  the  most  practical. 

For  convenience  in  testing,  suitable  scales  have  been 
arranged  like  the  one  suggested  by  Zeigler  (B  722),  of 
Philadelphia. 

The  following  table  from  Jackson  shows  at  a  glance  the 
relation  of  the  refracting  angle  and  the  prism-diopters  to  the 
centrad. 


ANGLE.      CENTRAD. 


1.06 

2.12 

3.18 
4-23 
5.28 
6.32 

7-35 
8.38 


PRISM- 
DIOPTERS. 

i. 

2. 

3- 

4- 

5- 

6.01 

7.01 

8.02 


ANGLE. 

9-39 
10.39 

"•37 
12.34 
13.29 
15.16 

19-45 
36.03 


CENTRAD. 

PRISM- 
DIOPTERS. 

9 

9.02 

10 

10.03 

ii 

11.03 

12 

12.04 

13 

13.06 

15 

15.11 

20 

20.26 

50 

54-62 

Burnett  (B  714,  715,  723),  who  wrote  on  this  sub- 
ject quite  extensively,  says  that  the  practical  value  of  the 
prism-diopter  is  demonstrated  by  the  fact,  probably  not  gen- 
erally known,  that  all  the  prisms  manufactured  in  the 
United  States  since  1895  have  been  measured  and  num- 
bered by  the  prism-dioptral  system,  and,  whether  we  recog- 
nize it  or  not,  we  are  using  prism-diopters  in  our  work  every 
day,  even  though  we  may  order  our  prisms  in  degrees  or 
centrads.  If  foreign  manufacturers  would  adopt  the  same 
plan,  this  discussion  of  differences  would  become  unnecessary. 

§  3.  In  What  Forms  are  Prisms  Arranged  or  Combined 


Ophthalmological  Prisms 


283 


with  Each  Other  ? — The  simplest  of  all  is  the  single  prism. 
The  effect  of  this  has  already  been  described,  but  as  it  is 
difficult  to  adjust  this  accurately  in  a  circular  frame,  some 
practitioners  prefer  square  prisms  set  in  square  frames  (Fig. 
194). 

As  it  was  found  to  be  slow  and  annoying  to  reach  to  the 
trial  case  for  each  separate  prism,  Noyes  placed  them  in 
series.  Sometimes  they  are  made  as  in  Fig.  195,  or  still 
more  conveniently  arranged  in  a  series  of  about  twenty 
together.  (Fig.  196). 

As  single  prisms  had  thus  been  placed  side  by  side  in 
every  conceivable  order,  another  principle  was  brought  into 
use,  and  has  also  undergone  various  modifications.  This  is 
to  revolve  one  prism  before  another  in  such  a  way  as  to 


FIG.  194. — Frames  for  square  prisms. 

obtain  a  greater  or  less  refractive  effect.  Probably  the  first 
one  to  adopt  this  plan  for  clinical  purposes  was  the  French 
optician  Crete,  but  the  mechanical  part  of  that  contrivance 
was  rather  cumbersome,  so  Risley  used  instead  two  small 
prisms  in  a  frame  which  could  be  placed  in  the  trial  case, 
but  which  rotated  one  before  the  other  in  a  similar  manner 
(Fig.  197).  To  obtain  a  slower  increase  of  weaker  effects, 
Jackson  added  still  a  third  prism  (Fig.  198).  In  this  category 
should  be  placed  also  the  two  prisms  which  we  find  in 
the  arrangement  suggested  by  Stevens,  and  called  by  him  a 
phorometer.  Here  the  two  prisms  are  not  placed  directly 
upon  one  another,  but  one  rotates  before  each  eye.  The 
principle  involved,  however,  is  the  same.  In  that  instru- 
ment a  prism  of  eight  degrees  is  placed,  with  the  apex 
up,  before  one  eye,  and  before  the  other  eye  is  another 
prism  in  the  reverse  position.  By  a  simple  cog  arrange- 


Ophthalmological  Prisms 


ment,  each  is  made  to  turn  so  that  the  refracting  angle  points 
upward,  outward,  or   in  any  direction  desired. 
While    this    is  an    extremely    convenient   ar- 
rangement   for    determining  the  static  condi- 
tion   of   the  eyes,    as    we  have  seen,    on  the 


FlG.  195. — Horizontal  prisms  in  series  (Noyes). 


FIG.  196. — 
Series  of 
prisms  as 

used  by  the 
author. 


Ophthalmological  Prisms 


285 


other  hand  the  prisms  are  not  sufficiently  strong  for  testing 
the  dynamic  condition  often  found. 
§  4.   Results  Produced  by  the  Combination  of  Prisms. — 

Of  course  it  is  easy  to  increase  the  strength 
of  prisms  by  adding  one  to  another,  keep- 
ing base  to  base  and  apex  to  apex.  Thus 
two  of  four  degrees  each,  applied  one  on 
the  other  in  this  manner,  give  us  for  prac- 
tical purposes  one  prism  of  eight  degrees, 
or  by  reversing  their  direction  the  two 
prisms  exactly  neutralize  each  other. 

The  question  is  more  complicated, 
however,  when  we  wish  to  know  the 
resultant  strength  of  one  such  prism 
revolved  at  a  given  angle.  Here  we  are 
brought  at  once  to  a  formula  which  it  is 
convenient  to  have  for  reference,  because 
the  calculations  deduced  from  it  give  us 
the  markings  which  we  find  on  all  revolv- 
ing prisms,  including  those  on  the  phoro- 
meter.  The  following  calculation  is  given 
in  full  because  it  is  proper  to  show  how 
this  important  formula  is  obtained,  and 
also  because  the  method  here  followed  is 
apparently  new. 

Let  the  angle  at  the  apex  of  the  prism  (Fig.  199) 
which  is  to  be  revolved  be  denoted  by  P,  and  the  angle 
through  which  the  prism  revolves  by  R.  It  is  assumed 
that  the  prism  is  revolved  in  such  a  manner  that  its 
edges  af,  ib,  and  cd  describe  planes  perpendicular  to 
the  plane  in  which  we  wish  to  measure  the  component 
of  refraction.  It  is  also  assumed  for  the  sake  of  conven- 
ience that  we  are  dealing  with  a  right-angled  prism, 
and  that  the  right  angle  is  formed  by  the  planes  ci 
and  ab,  and  that  the  three  edges  ji,  eh,  and  af  of  the 
prism  are  parallel.  Let  us  now  imagine  the  revolving 
prism  rotated  from  its  horizontal  position  on  the  point 
f  in  space  as  an  axis,  where  the  apex  af  is  perpen-, 
dicular  to  the  plane  in  which  we  wish  to  measure  the 
component  of  refraction.  If  then  we  conceive  this 
plane  as  marking  off  a  section  of  the  prism  it  would 


FIG.  197. — Vertical 
prisms  arranged  in 
series,  (natural  size.) 


286  Combination  of  Prisms 

mark  a  triangle  which,  when  revolved  to  the  position  in  which  we  wish  to 
measure  the  equivalent  prism,  is  the  triangle  jfe.  Since  the  plane  of  the 
triangle  is  perpendicular  to  af,  the  lines  fj  and  fe,  in  the  planes  aj  and  ae, 
measure  the  angle  of  the  prism;  hence  the  angle  jfe  =  P. 

We  now  revolve  our  prism  to  the  position  in  which  we  desire  to  find  an 
equivalent  prism,  and  then  let  the  plane  in  which  we  propose  to  measure  the 
component  of  refraction  again  pierce  the  prism.  It  will  cut  a  triangle  hfi.  Let 
us  suppose  that  we  now  pass  planes  through  the  lines  fi  and  hf,  and  that  these 
planes  are  perpendicular  to  the  plane  in  which  we  desire  to  measure  the  com- 
ponent of  refraction;  then  we  have  here 
a  prism  whose  actual  plane  of  refraction 
coincides  with  the  plane  in  which  we  are 
going  to  measure  the  component  of  refrac- 
tion, hence  our  revolving  prism  in  its 
position  is  equivalent  to  this  new  prism 
and  all  we  must  do  is  to  measure  the 
angle  hfi  at  its  apex,  which  we  shall  call 
P1.  Furthermore,  the  angle  jfi  through 
which  the  prism  has  revolved,  we  shall 
call  the  angle  R.  Since  the  planes  in 
which  the  lines  bi  and  ch  revolve  are 
FIG.  198. — Rotating  prisms  (Risley)  perpendicular  to  our  plane  of  refraction, 

je  and  hi,  which  lie  in  the  same   plane, 

are  perpendicular  to  bi,  hence  they  are  parallel,  and  since  eh  and  ji  are 
parallel,  je  and  hi  are  equal.  Furthermore,  the  angles  ejf  and  hif  are  right 
angles. 

sin  P  =-J^,  hence  je  =  fe  sin  P 
fe 

fe  fe 

cos  R  =  _'   hence  fh  =  ;— 

f h  cos  R 

hi 

sm  p  =  7h 

Substituting  for  hi  its  equal  je, 

sin  P'  •=  }.—     Substituting  for  je  and  fh  their  respective  values, 
fh 

fe  sin  P       fe  sin  P  cos  R         .     v  ^^  T> 

sm   P  =  — =  —     — —          =  sm  f  cos  K. 

fe  fe 

cos  R 
P'  =  sin   J  (sin  P  cos  R) 

In  like  manner  if  we  denote  by  R'  the  angle  through  which  the  other  prism 
revolves  from  the  plane  in  which  we  wish  to  measure  the  component  of  refrac- 
tion, by  Q  the  angle  of  the  apex  of  the  prism,  and  by  Q'  its  equivalent 
angle  after  rotation,  then 

Q'  =  sin-1  (sin  Q  cos  R') 

Denoting  by  S  the  sum  of  the  equivalent  prisms,  S  =  P'  +  Q'  =  sm~'  (sin 
P  cos  R)  -j-  sin"1  (sin  Q  cos  R'). 

Let  us  propose  to  ourselves  a  few  practical  problems  as,  given  a  prism  of  8° 


Combination  of  Prisms 


287 


revolved  upward  through  10°,  and  a  prism  of  9°  revolved  downward  15°,  then 
our  formula  becomes 

S  =  sin"1  (sin  8°  cos  10°)  -f-  sin~'  (sin  9°  cos  —  15°) 

where  15°  has  a  minus  sign  before  it,  since  we  have  arbitrarily  assumed  that 
angles  of  rotation  made  by  revolving  the  prisms  upward  are  considered  positive 
and  those  downward  negative. 

log  sin  8°  =  9.14356 
log  cos  10°  =  9.99335 
log  sin  P'  ='  9.13691 

where  P'  is  defined  by  the  equation  sin  P'  =  sin  8°  cos  10°  and  P'  =  sin"1  (sin 
8°  cos  10°). 


FIG.  199. — One  prism  revolving  on  another. 

Looking  in  the  table  for  an  angle  the  logarithm  of  whose  sin  is  9.13691,  we 
find  P'  =  7°  52'. 66. 

For  the  last  part  of  our  equation 

log  sin  9°    =9.19433 

log  cos  15°  =  9.98494 

log  sin  Q'    =  9.17927 

where  Q'  is  defined  by  the  equation  sin  Q'  =  sin  9°  cos  15°  and  Qf  SB  rin~l 
(sin  9°  cos  15°). 


288  Combination  of  Prisms 

Looking  in  the  table  for  an  angle  the  logarithm  of  whose  sin  is  9.17927,  we 
find  Q'  =  8°  41'. 45,  S  =  P'  -f  Q'  =  7°  52'.66  +  8°  4i'.45  =  16°  34  .11. 

For  another  problem  let  us  assume  that  one  prism  has  an  angle  of  10°  and  is 
revolved  upward  through  5°,  and  that  attached  to  it  is  another  prism  of  8°  re- 
volved upward  through  120°,  here  P  =  10°,  R  =  5°,  Q  =  8°,  R'  =  120°. 

S  =  sin-1  (sin  10°  cos  5°)  +  sin  ~l  (sin  8°  cos  120). 
log  sin  10°  =  9.23967 
log  cos  5°    =  9.99834 
log  sin  P'    =  9.23801 

where  P'  is  defined  in  a  manner  similar  to  that  above,  and  equals  9°  57'.6g. 
Regarding  the  second  part  of  our  equation,  the  log  cos  120°  is  not  given  in  the 
tables  and  we  must  find  an  equivalent  expresssion. 

cos  120°  =  cos  (180°  —  6o°)=  —  cos  60° 
sin-1  (sin  8°  cos  120°)   =  sin-1  ( -j  sin  8°  I    j  —  cos  60°  I )=  sin-1—!  (sin  8° 

cos  60"  >• ) 

log  sin  8°  =  9.14356 
log  cos  60°  =  9.69897 
log  sin  Q'  =  8.84253 

where  Q'  is  defined  by  the  equation,  sin  Q'  =  sin  8°  cos  60°,  whence  sin  —  Q 
=  —  sin  8°  cos  60°  and —  Q'  =  sin"1  ( —  sin  8°  cos  60°). 

Looking  up  the  angle  the  logarithm  of  whose  sin  is  8.84253  we  find  Q'  = 
3°  59'. 42,  but  the  equivalent  of  the  expression  sin"1  ( — sin  8°  cos  60°)  is  —  Q' 
hence  our  angle  is  —  3°  59'. 42.  From  above 

S  =  sin  -1  (sin  10°  cos  5°)  +  sin-1  (sin  8°  cos  120°) 
=  P  +  ( ~  Q) 

=  9°  57'.69  —  3°  5g'.42  =  5°  58'.  27 

The  truth  of  these  formulas  is  apparent  if  we  consider  a  few  simple  problems 
of  which  we  know  the  answer.  Suppose  the  two  prisms  are  equal  and  we  re- 
volve one  through  180°,  keeping  the  other  fixed,  then  the  two  prisms  ought  to 
neutralize  each  other.  Let  us  see  if  they  do.  Here, 

P  =  Q,  R'  =  o°,  R  =  180° 

S  =  sin"1  (sin  P  cos  180°)  -f-  sin  ~]  (sin  P  cos  o°) 

cos  180°,  =  —  I,  cos  o°  =  -f-  i 

S  =  sin  -1  (  —  sin  P)  +  sin  "'  (sin  P) 

but  —  sin  P  =  sin  —  P,  hence  sin"1  ( —  sin  P)  =  sin1  (sin  —  P)  =  —  P,  and 
sin"1  (sin  P)  =  -\-  P,  hence 

S  =  —  P+P  =  o 


Actual  Deflection  Produced  by  a  Prism        289 


If  one  prism  is  held  fixed  as  before  and  the  other  revolved  through  90°,  then 
we  have  the  effect  of  one  prism.  Here, 

S  =  sin"1  (sin  P  cos  90°)  +  sin  "'  (sin  P  cos  o°) 
but  cos  90°  =  o,  and  cos  ou  =  i,  hence 

S  =  sin  ~J  (o)  +  sin~l  (sin  P)  =  o  +  P  =  P. 

§  5.  What  is  the  Actual  Deflection  Produced  by  a 
Prism  ? — In  the  right-angle  triangle  ABC,  (Fig.  200)  the 
distance  AB  being  always  100  centimeters,  the  line  BC  rep- 
resents the  amount  of  deflection  caused  by  a  prism  placed  at 
the  point  A  in  the  position  of  minimum  deviation.  Thus 
if  a  prism  of  5  degrees  were  to  cause  a  displacement  of 
4.5  centimeters  along  such  a  line,  then  the  tangent  of 
the  angle  between  the  normal  and  refracted  rays  would  be 
found  simply  by  dividing  the  length  of  the  opposite  side  by 
the  length  of  the  adjacent  side,  that  is  -£^.  In  the  table  of 
natural  tangents  this  corresponds  to  an  angle  of  2°  34'.  Thus 

TABLE  OF  NATURAL  TANGENTS 


Angle  of  the 
prism. 

Linear  deflection  in 
centimeters. 

Angle  to  which  the 
linear  deflection  is 
tangent. 

Same  in  decimals. 

I 

0.91 

o°  31'.  20 

0.502° 

2 

1.82 

1°  02'.42 

1.040° 

3 

2.72 

i°  33'.  62 

1.560° 

4 

3-63 

2°  04'.  92 

2.084° 

5 

4-55 

2°  36'.  25 

2.604° 

6 

5.46 

3°  o?'.65 

3-013° 

7 

6.38 

3°  39'-°9 

3-651° 

8 

7.30 

4°  io'.66 

4-178° 

9 

8.23 

4°  42'.  29 

4-705° 

10 

9.16 

5°  I4'.<>7 

5-235° 

ii 

10.  IO 

5°  45'-98 

5-766° 

12 

11.04 

6°  i8'.02 

6.300° 

13 

11.99 

6°  50'.  19 

6.837° 

14 

12.95 

7°  22'.58 

7.376° 

15 

13-Qi 

7°  55'.i3 

7.919° 

16 

14.88 

8°  27'.86 

8.464° 

17 

15.87 

9°  oo'.87 

9.014° 

18 

16.86 

9°34'.« 

9-569" 

*9 

17.86 

10°  07'.  56 

10.126* 

20 

18.87 

10°  41'.  32 

10.688° 

it  is  evidently  easy  to  construct  a  table  of  these  deflections. 
In  this,  the  first  column   to   the  left  gives  the   angles  of 
the  prism  as  measured  by  the  goniometer  or  otherwise,  the 
19 


290        Prismatic  Effect  of  Decentered  Lenses 


second  column  shows  the  amount  of  linear  deflection  pro- 
duced, and  the  third  gives  the  tangents  of  the  angles.  Al- 
though such  a  table  is  simple  enough,  a  search  through  the 
literature  showed  that  it  had  been  prepared  only  once. 
That  was  by  Bisinger  (B  778,  p.  72).  But  unfortunately 
his  figures  are  wrong.  For  on  proving  the  angles  with 
a  table  of  natural  tangents,  it  appears  that 
he  gives  the  sine  of  the  angle,  when  he 
himself  says  it  should  be  the  tangent.  Of 
course,  these  figures  are  subject  to  slight 
variations,  due  to  the  difference  in  the 
density  of  the  glass,  but  simple  as  the 
table  is,  it  is  worth  while  to  correct  it  and 
give  it  for  the  convenience  of  those  who 
otherwise  may  look  for  it  in  vain.  More- 
over, such  a  table  is  frequently  needed  in 
the  conversion  of  meter  angles  into  de- 
grees, and  the  reverse.  Later  we  shall  see 
the  importance  of  this,  when,  in  the  patho- 
logical part  of  our  study,  we  express 
degrees  of  muscle  imbalance  in  terms  as 
exact  as  possible. 

-§6.  Prismatic  Effect  Produced  by 
Decentering  Lenses.—  In  clinical  terms 
we  say  that  a  lens  is  decentered  when  its 
axis  does  not  coincide  with  the  optic  axis 
of  the  eye  before  which  it  is  placed.  When 
FIG.  200.—  Deflection  tnat  occurs  a  prismatic  effect  is  produced, 
caused  by  a  prism,  as  can  be  easily  seen  by  moving  a  convex 
or  concave  lens  before  the  eye.  The 
amount  of  prismatic  effect  produced  by  a  given  amount  of 
decentering  is  evidently  dependent  upon  two  factors,  the 
strength  of  the  glass,  and  the  distance  which  the  optic  axis 
and  the  axis  of  the  lens  are  separated  from  each  other.  It 
is  not  difficult  to  obtain  a  formula  which  will  express  pro- 
perly the  relation  of  these  two  factors  to  each  other.  Opti- 
cians are  accustomed  to  figure  that  a  lens  which  is  decentered 
one  centimeter  will  produce  as  many  prism-diopters  as  it 
has  diopters  of  refraction.  Thus,  that  if  a  lens  of  five  diop- 


B 


Prismatic  Effect  of  Decentered  Lenses       291 


ters  is  decentered  one  centimeter,  it  will  produce  a  dis- 
placement of  five  prism-diopters ;  and  when  it  is  decentered 
two  centimeters  it  will  produce  a  displacement  of  ten  prism- 
diopters,  etc.  The  following  is  a  table  showing  more  exactly 
the  number  of  millimeters  which  it  is  necessary  to  decenter 
a  spherical  lens  in  order  to  add  a  prism  of  from  i°  to  5°. 


+     or  —  glass 
of 

Strength  of  Prism. 

I 

2 

3 

4 

5 

0.25 

0.50 

18.5 

0.75 

12.3 

I.OO 

9.2 

I8.5 

1-25 

7-4 

14.8 

22.2 

1.50 

6.2 

12.3 

I8.5 

1.75 

5.3 

10.6 

'5-9 

2I.I 

2.OO 

4.6 

9.2 

13-9 

I8.5 

2.25 

4-1 

8.2 

12.3 

16.4 

20.5 

2.50 

3-7 

7.4 

u.  i 

14.8 

18.5 

2-75 

3-4 

6.7 

10.  1 

13-4 

16.8 

3.00 

3-1 

6.2 

9.2 

12.3 

15-4 

3-25 

2.8 

5-7 

8.5 

11.4 

14.2 

3-50 

2.6 

5-3 

7-9 

10.6 

13.2 

4.00 

2-3 

4.6 

6.9 

9-2 

U-5 

4-50 

2.1 

4.1 

6.2 

8.2 

10.3 

5.00 

1.8 

3-7 

5-5 

7-4 

9.2 

5-50 

1-7 

3-4 

5-0 

6.7 

8.4 

6.00 

1-5 

3.1 

4.6 

6.2 

7-7 

Evidently  it  is  unnecessary  to  calculate  the  distances  which 
the  weaker  glasses  must  be  decentered  to  equal  the  weaker 
prisms,  because  we  soon  find  that  the  amount  of  decentering 
is  far  greater  than  the  entire  width  of  an  ordinary  spectacle 
or  eye-glass. 


DIVISION  II. 
Convergence. 

§  i.  Convergence  and  Definition  of  the  Meter  Angle. — 

Convergence  consists  in  an  angular  motion  of  the  visual  axes 
toward  each  other.  In  the  normal  condition,  both  accom- 
modation and  torsion  keep  pace  approximately  with  the  de- 
gree of  convergence,  the  amount  being  in  proportion  to  the 
size  of  the  angle  which  the  visual  axes  make  with  each  other. 
The  earlier  ophthalmologists  were  accustomed  to  estimate 
convergence  in  degrees  of  that  angle,  but  after  the  metric 
system  had  been  brought  into  ophthalmology  at  Nagel's  sug- 
gestion (B  704)  it  occurred  to  him  to  express  convergence  also 
in  terms  of  what  he  designated  the  "  meter  angle."  This  we 
understand  as  the  amount  of  convergence  required  for  the 
visual  axes  of  a  given  individual  to  meet  at  a  point  situated 
one  meter  distant  from  the  center  of  the  line  which  connects 
the  center  of  motion  of  the  two  eyes — that  is,  from  the  center 
of  the  base  line.  Thus  (Fig.  201)  if  OO  is  the  distance 
between  the  centers  of  motion  of  the  two  eyes  and  MR  a 
perpendicular  one  meter  long,  erected  at  the  middle  of  that 
line,  then  the  ajigle  QOR,  or  its  equivalent  ORM,  is  called 
one-meter  angle.  If  the  visual  axes  cross  at  a  point  twice 
as  near  to  the  eye — that  is,  at  half  a  meter  distant,  then  the 
angle  of  convergence  is  said  to  be  two-meter  angles,  and  so 
on.  This  method  of  expressing  the  amount  of  convergence 
in  terms  of  a  meter  is  not  only  ingenious  but  convenient,  and 
its  clinical  advantage  is  well  known. 

§  2.  Meter  Angles  Expressed  in  Degrees.— The 
question  is :  Given  the  length  of  the  base  line  and  the  angle 
of  convergence  expressed  in  meter  angles,  what  is  the  size  of 
that  angle  when  expressed  in  degrees  and  minutes? 

In  order  to  find  the  angle  of  convergence  in  Fig.  201  let 

292 


Meter  Angles  Expressed  in  Degrees         293 


OM  =  d,  and  the  distance  of  the  object  =  n ;  let  C  =  the 
angle  which  each  eye  converges. 

Then  sin  C  =  — 
n 

For  example,  if  the  distance  between  the  eyes  in  a  given 
individual  is  58  millimeters  and  the  distance  of  the  object 

(n)  =  J  meter,  then  the  sine  of  the  meter  angle  =  2g  mm     = 

250  mm 

.11600,  and  we  find  from  a  table  of  natural  sines  that  this 
decimal  corresponds  to  the  sine  of  an  angle  of  6°-4O/. 

Nowhere  in  the  literature  was  there  a  table  of  the  angles 


O  d  M  O 

FIG.  201. — The  meter  angle. 

at  which  eyes  converge  for  all  possible  lengths  of  the  line 
OO'.  Nagel's  calculations,  which  cover  all  variations  in  the 
base  line  from  55  to  75  millimeters,  are  based  on  a  conver- 
gence of  one-meter  angle  only,  while  those  for  degrees  of 
convergence  from  one-meter  angle  up  to  twenty  are  only 
for  a  person  with  a  base  line  of  64  millimeters.  These  two 
sets  of  calculations  have  been  copied  in  Landolt,  Norris  and 
Oliver,  and  in  other  text-books.  But  they  cover  only  a  very 
limited  range.  It  seemed  worth  while  therefore  to  complete 
this  table  (B  810).  The  first  column  on  the  left  gives  the 


294         Meter  Angles  Expressed  in  Degrees 


0 

M 

^  t^oo  o  o  M  m 

o  mo  m  «  •«•  M 

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\rt 

"«     0     0>  CXVO     IT,   * 

me*  MMOOOOOOMN  nio 

M    N    0    N    IN    «    « 

M«MRftaaRftasft 

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t--. 

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O  w  co  ^  »o  o  w 

^^ss'ss-^^sy^^s^ 

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s^VsVsVaVVsVK 

Degrees  Expressed  in  Meter  Angles         295 

length  of  the  base  line  in  millimeters,  while  the  upper 
horizontal  line  shows  the  number  of  meter  angles  of  conver- 
gence. Knowing  these,  the  corresponding  angle  of  con- 
vergence in  degrees  and  minutes  can  be  seen  at  a  glance. 

§  3.  Degrees  of  Convergence  Expressed  in  Meter 
Angles. — This  question  is :  Given  the  length  of  the  base 
line  and  the  angle  of  convergence  expressed  in  degrees, 
what  is  the  size  of  that  angle  when  expressed  in  meter 
angles  ? 

This  deserves  a  place  in  our  study  not  only  because  it  is  in 
part  a  restatement  of  the  problem  which  has  just  been 
considered,  but  because  of  the  very  practical  use  which  can 
be  made  of  the  data  thus  obtained.  The  calculation  is  easy, 
depending  simply  on  a  change  of  the  formula  already  em- 
ployed. 

The  meter  angle  =  - 

Again  if  C  =  angle  which  each  eye  converges,  then 

_       d         I         sin  C 
sin  C  =  —  or  —   =  — : — 
n        n          d 

In  this  we  have  given,  d  (one  half  the  base  line)  and  sin  C. 
This  gives  —  in  meter  angles  when  d  is  expressed  in  meters. 

For  example,  in  a  person  with  a  base  line  of  62  millime- 
ters and  a  convergence  of  6°, 

sin  6°  =   ai°453  =  3<37  meter  angles, 
d  0.03 1 

The  results  are  found  in  the  accompanying  table.  The  line 
above  represents  the  length  of  the  base  line  expressed  in  milli- 
meters extending  from  fifty-five  to  sixty-six.  The  first 
vertical  column  on  the  left  gives  the  size  of  the  angle  of  con- 
vergence of  one  eye— that  is,  the  angle  which  one  visual  axis 
makes  with  the  perpendicular  erected  at  the  center  of  the 
base  line  M.  It  is  the  angle  ORM  or  OFM,  etc.  This  angle 
is  calculated  from  one  degree  to  twenty  for  each  eye,  or  from 
two  to  forty  degrees,  of  course,  for  both,  the  figure  then  in 
each  of  the  vertical  columns  represents  the  size  of  this  angle 
in  terms  of  the  meter  angle. 


296         Degrees  Expressed  in  Meter  Angles 


55 


Base  Line  in  Millimeters 
56        57         58         59       60        61        62       63        64        65      66 


1—  (    - 

I 

0.63 

O.62 

O.6l 

O.6O 

0.59 

0.58 

o.57 

0.56 

0-55 

0.54 

0.54 

0.53 

2 

1.27 

1-25 

1.22 

1.  2O 

1.18 

1.16 

1.14 

1.  12 

i.  ii 

1.09 

1.07 

i.  06 

3 

1.90 

1.87 

1.84 

1.  80 

1-77 

1-74 

1.72 

1.69 

1.66 

1.64 

1.61 

1.59 

4 

2.54 

2.49 

2-45 

2.40 

2.36 

2  32 

2.29 

2.25 

2.21 

2.18 

2.15 

2.II 

5 

3.17 

3." 

3.06 

3.00 

2.95 

2.90 

2.36 

2.8l 

2.77 

2.72 

2.68 

2.64 

6 

3.80 

3-73 

3.67 

3  60 

3-54 

3.48 

3-43 

3-37 

3-32 

3-27 

3.22 

3-17 

7 

4-43 

4-35 

4.28 

4.20 

4-13 

4.06 

4.00 

3-93 

3-87 

3-8i 

3-75 

3.69 

8 

5.06 

4-97 

4.88 

4.80 

4.72 

4.f4 

4.56 

4-49 

4.42 

4-35 

4.28 

4.22 

9 

5.69 

5-59 

5-49 

5-39 

5-30 

5-21 

5.13 

5-05 

4-97 

4-89 

4.81 

4-74 

10 

6.31 

6.20 

6.09 

5-99 

5-89 

5-79 

5.69 

5.60 

5-51 

5-43 

5-34 

5.26 

ii 

6.94 

6.81 

6.70 

6.58 

6.47 

6.36 

6.26 

6.16 

6.06 

5-96 

587 

5.78 

12 

7.56 

7-43 

7.30 

7.17 

7-05 

6.93 

6.82 

6.71 

6.60 

6.50 

6.40 

6.30 

13 

8.18 

8.03 

7.89 

7.56 

7.63 

7-50 

7.38 

7.26 

7.14 

7.03 

6.92 

6.82 

14 

8.80 

8.64 

8.49 

8-34 

8.20 

8.06 

7-93 

7.80 

7.68 

7.56 

7-44 

7-33 

15 

9.41 

9.24 

9.08 

8.92 

8.77 

8.63 

8.49 

8.35 

8.22 

8.09 

7.96 

7.84 

16 

IO.O2 

9.84 

9-67 

9.40 

9-34 

9.19 

9.04 

8.89 

8.75 

8.61 

8.48 

8.35 

17 

0.63 

10.44 

10.26 

10.08 

9.91 

9-74 

9.58 

9-43 

9.28 

9.14 

9.00 

8.86 

18 

11.24 

11.04 

10.84 

10.66 

10.48 

10.30 

10.13 

9-97 

9.81 

9.66 

9-51 

9-36 

19 

11.84 

11.63 

11.42 

11.23 

11.04 

10.85 

10.67 

10.50 

10.33 

10.17 

IO.O2 

9.86 

20 

12.44 

12.22 

12.00 

11.80 

TT.6O 

11.40 

11.22 

11.03 

10.86 

10.69 

10.52 

10.37 

It  may  naturally  be  asked  why  we  devote  so  much  time  to 
the  calculation  of  this  table,  or  of  what  value  it  is  when 
finished  ?  In  the  first  place,  it  seems  always  worth  while  thus 
to  complete  by  mathematical  data  the  physiological  basis 
on  which  our  knowledge  of  the  muscles  must  rest.  But  this 
table  has  another  and  more  immediate  value.  It  helps 
us  to  translate  into  modern  terms  the  data  which  were 
found  by  the  earlier  students,  especially  those  who  made 
their  calculations  before  Nagel  introduced  the  meter  angle 
into  ophthalmology.  The  result  is  that  the  measurements  of 
Helmholtz  (B  584),  Bonders  (B  590),  Hering  (B  586),  and 
Landolt  (B  592)  are  practically  lost  to  us  because  we  cannot 
express  them  in  terms  of  a  meter  angle  without  a  tedious 
calculation  each  time.  But  this  table  permits  that  trans- 
position to  be  made  at  a  glance.  Moreover,  we  shall  see 
that  these  earlier  measurements  of  convergence,  in  their  rela- 
tion to  accommodation  or  to  torsion — data  which  have  lain 
forgotten  in  Graefes  Archives  for  a  third  of  a  century  or  more 
— prove  to  be  of  decided  clinical  value  in  the  light  of  modern 


Relative  or  Fusion  Power  297 

methods  of  investigation.  Finally,  this  table  enables  us  to 
express  also  in  meter  angles  the  result  of  our  examinations 
with  prisms  as  to  the  static  and  dynamic  conditions  of  con- 
vergence. Thus  an  esophoria  of  five  degrees  means  that  the 
visual  axis  of  the  eye  tested  tends  to  make  a  certain  definite 
angle  with  a  perpendicular  to  the  center  of  the  base  line  in 
the  horizontal  plane,  and  as  we  can  easily  find  the  base  line 
by  methods  already  given,  we  know  at  once  the  fraction  of  a 
meter  angle  which  that  esophoria  would  represent  if  it  were 
a  corresponding  esotropia. 

As  we  are  able  to  represent  accommodation  by  a  line 
divided  into  equal  parts,  each  one  of  which  corresponds  to 
one  diopter,  so  can  we  also  represent  convergence  by  a  line 
divided  into  equal  parts,  each  one  of  which  corresponds  to 
one  meter  angle  of  convergence.  In  other  words,  accommo- 
dation is  made  to  accord  with  convergence,  and  that,  in  turn, 
in  a  certain  way  with  forms  of  heterotropia.  Later  we  shall 
find  that  in  a  similar  manner  torsion  may  be  expressed 
diagrammatically.  This  gives  us,  as  we  shall  see,  a  manner 
of  representing  the  relation  of  accommodation,  of  con- 
vergence and  of  torsion  to  each  other.  In  other  words,  these 
tables  assist  us  in  bringing  our  scattered  data  into  relation 
with  each  other. 

§  4.  What  is  the  Relative  or  Fusion  Power  ? — In  a  pre- 
vious chapter,  when  studying  the  motions  of  one  eye  alone, 
we  found  that  the  adductor  group  could  exert  a  sufficient 
force  to  lift  a  certain  weight  and  the  energy  expended  in 
doing  this  was  called  the  lifting  power  or  absolute  power  of 
adduction. 

But  the  power  of  the  adductor  muscles  can  be  exerted  in 
another  way — that  is,  by  placing  prisms  base  out  before 
one  eye  or  both ;  then,  in  the  effort  to  avoid  double 
vision,  one  eye  or  both  turn  in.  As  this  effort  is  one 
exerted  in  relation  to  the  opposing  groups  of  muscles,  we 
can  properly  call  it  the  relative  power  of  adduction.  Its 
object  is  to  fuse  the  retinal  images.  While  this  relative  or 
fusion  power  of  adduction  is  to  be  clearly  distinguished 
from  the  absolute  or  lifting  power  of  that  group,  as  the 
latter  has  thus  far  been  studied  so  little,  we  will  understand 


298  Measurement  of  Fusion  Power 

in  the  future  that  the  relative  or  fusion  power  is  referred  to, 
unless  the  contrary  is  stated.  It  depends  upon  two  factors, 
the  actual  strength  of  the  recti  muscles  and  the  so-called 
instinctive  desire  of  fusion.  Both  of  these  vary  in  different 
individuals.  Although  we  are  confining  our  attention  now 
to  the  power  of  adduction  in  overcoming  prisms,  if  we 
remember  that  the  same  principle  applies  to  abduction, 
superduction,  subduction,  etc.,  much  repetition  can  be 
avoided. 

How  is  this  Relative  or  Fusion  Power  Measured  ? — 
Although  this  is  a  subject  already  perfectly  familiar  to  most 
readers,  it  is  worth  while  to  review  it  here  in  order,  if 
possible,  to  clear  up,  from  the  physiological  standpoint,  the 
confusion  which  exists  clinically.  To  begin  with  an  ele- 
mentary statement :  If  the  parallel  visual  axes  are  directed 
toward  a  distant  object — a  point  of  light,  for  example — and 
a  prism  is  held  before  the  right  eye  with  the  base  out, 
then,  as  the  ray  from  the  light  passes  through  the  prism 
and  is  deflected  towards  its  base,  the  image  of  the  light 
falls,  not  on  the  fovea  of  the  right  eye,  but  on  its  outer 
side,  and  crossed  diplopia  results.  Whenever  such  diplopia 
occurs,  there  is  an  instinctive  desire  to  overcome  it,  and 
immediately  the  eye  tends  to  turn  inward  to  meet  the 
ray  thus  deflected  outward.  If  the  prism  is  a  weak  one, 
the  adductor  muscles  turn  the  eye  far  enough  inward  to 
overcome  the  diplopia.  With  a  stronger  prism,  the  eye 
turns  in  still  more,  and  again  more,  but  the  limit  of  the 
ability  of  the  abducting  muscles  of  each  eye  to  turn  the  globe 
toward  the  median  line  is  finally  reached,  and  the  strong- 
est prism  which  the  abductor  muscles  can  thus  "  over- 
come "  is  said  to  represent  the  power  of  adduction  of  that 
pair  of  eyes.  Half  the  strength  of  the  prism  would  be  the 
adductive  power  of  one  eye.  In  a  similar  way  we  measure 
the  power  of  abduction  by  turning  the  prism  with  the  apex 
outward,  or  of  superduction  and  subduction  by  turning  the 
apex  down  and  up.  These  facts  are  part  of  the  basal 
knowledge  of  every  ophthalmologist.  Unfortunately,  how- 
ever, we  are  very  far  from  agreeing  on  the  interpretation 
of  the  data  obtained  by  this  very  simple  procedure. 


Measurement  of  Fusion  Power  299 

We  have  most  confused  statements  of  how  the  power 
of  adduction  should  be  measured,  what  its  amount  is 
in  a  normal  condition,  what  its  value  is  clinically,  and,  in- 
deed, whether  such  examinations  have  any  value  at  all.  Yet 
these  are  all  important  questions,  and  it  is  necessary  to  estab- 
lish them  clearly  on  a  physiological  basis  or  we  shall  continue 
to  flounder  in  a.  confusion  of  methods  and  theories.  If  we 
turn  first  to  our  methods  of  making  the  examinations,  that 
will  perhaps  indicate  in  what  one  part  of  the  trouble  lies. 
Let  us  begin  by  placing  a  prism  of  five  degrees,  base  out, 
before  the  right  eye.  Suppose  the  eye  overcomes  this  prism 
and  others  of  gradually  increasing  strength,  as  they  are 
selected  from  the  test  case,  until  we  find  at  last  that  one  of 
nine  degrees  represents  the  total  adductive  power.  Or  we 
may  begin  with  prisms  of  twelve  or  fourteen  degrees 
taken  again  from  the  test  case,  and,  decreasing  from  that 
point,  find  again  that  one  of  nine  degrees  is  the  strongest 
which  the  adductor  muscles  can  overcome.  -If,  however,  we 
vary  that  method  of  testing  by  gradually  increasing  the 
strength  of  the  prism  without  allozving  the  eye  an  interval  in 
which  to  rest,  the  result  is  usually  different.  If  we  use  the 
prisms  in  series  of  Noyes  or  others  like  them,  the  amount  of 
adduction  can  be  brought  to  ten  or  twelve,  and  if  we  use 
Risley's  prisms  or  some  of  the  other  rotating  prisms  in 
which  the  increment  is  still  more  gradual  and  without 
interruption,  the  adductive  power  in  the  same  individual  at 
the  same  sitting  can  be  brought  perhaps  to  fifteen  or 
eighteen,  or  in  occasional  instances  to  a  much  greater 
number  of  degrees  proportionately.  Evidently,  then,  we 
have  different  results  dependent  upon  different  methods  of 
testing.  At  present  we  confuse  these.  They  are  certainly 
distinct  physiologically,  and  should  be  distinguished  clinically. 
For  convenience,  we  could  classify  them  as  follows: 

First.  The  minimum  relative  or  fusion  power  of  a  group  of 
muscles  is  that  which  we  obtain  by  placing  different  prisms 
before  the  eye,  leaving  a  considerable  interval  between 
the  tests,  or  it  is  that  which  is  found  when  we  pass  from 
a  prism  strong  enough  to  produce  diplopia  to  one  which  can 
be  overcome. 


300  Minimum  Fusion  Power 

Second.  The  maximum  relative  or  fusion  power  of  a  group 
of  muscles  is  that  which  we  obtain  when  we  pass  by  gradual 
increment  from  a  prism  which  is  not  strong  enough  to  pro- 
duce diplopia  to  one  which  cannot  be  overcome. 

Third,  as  confusion  and  misunderstandings  often  occur 
in  recording  the  results  of  any  such  tests,  we  should  specify 
what  is  meant  by  the  figure  used.  If  it  is  the  strength  of 
one  prism  or,  what  is  the  same  thing,  if  it  is  the  sum  of 
the  strength  of  the  prisms  held  before  each  eye,  then  we 
may  properly  call  that  the  total  minimum  (or  maximum) 
power  of  adduction  or  abduction,  etc.  If,  however,  the 
figure  used  expresses  only  half  of  the  sum  of  the  strength 
of  the  prisms  held  before  each  eye,  then  that  fact  should 
be  clearly  stated. 

The  difference  between  the  minimum  and  maximum 
power  of  any  group  of  muscles,  especially  in  adduction  and 
abduction,  can  ordinarily  be  found  at  once  on  making 
this  simple  test.  In  some  persons  it  is  true  that  there  is 
practically  no  difference,  while  in  others  there  is  a  wide  range 
of  from  six,  eight,  to  ten  degrees  or  more. 

The  importance  of  this  distinction  is  great.  Until  it  is 
made,  and  until  we  agree  upon  some  uniform  method  for 
this  portion  of  our  clinical  work,  we  do  not  understand  each 
other. 

§  5.  What  is  the  Minimum  Fusion  Power  of  the  Various 
Groups  of  Muscles? — In  this  division  of  the  fourth  group 
of  movements,  we  are  dealing  in  a  strict  sense  only  with 
convergence.  But  divergence  is  really,  as  Landolt  has  called 
it,  a  minus  convergence.  Therefore  we  can  with  advantage 
glance  at  divergence  also  in  this  connection.  Or,  if  we 
include  at  this  point  efforts  to  fuse  images  in  the  verti- 
cal meridian,  it  will  be  unnecessary  to  refer  to  them 
again. 

Different  methods  have  been  employed  for  testing  normal 
and  abnormal  eyes  and  naturally  the  results  appear  contra- 
dictory. An  idea  of  this  confusion  can  be  obtained  by 
arranging  the  observations  of  a  few  American  writers  in 
tabular  form,  as  on  the  next  page.  This  shows  the  amount 


Minimum  Fusion  Power 

of  adduction,  abduction,  etc.,  when  the  test  object  is 
distance  of  six  meters. 


301 
at  a 


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2. 

As  it  seemed  probable  that  the  varying  results  which  were 
obtained,  at  least  for  adduction,  were  due  partly  to  the  fact 
that  some  observers  included  examinations  of  abnormal 
eyes,  and  as  nearly  all  employed  different  tests,  it  appeared 
to  me  worth  while  to  go  over  this  ground  again.  Accord- 
ingly an  examination  as  to  this  point  was  made  of  56  soldiers 
at  Fort  Porter,  of  31  Harvard  students,  and  16  pairs  of  non- 
asthenopic  eyes  among  my  colleagues  and  other  friends,  or 
a  total  of  103  individuals.  With  all  of  these,  the  prisms 
were  used,  as  already  described,  so  as  to  obtain  only  the  mini- 
mum fusion  power.  The  results  were  as  follows  :  Adduc- 
tkm  9.7  ;  abduction  6.8  ;  superduction  2.3  ;  subduction  2. 

It  is  probable  that  these  figures  will  need  some  revision 
not  only  because  the  number  of  persons  was  small,  but 
apparently  the  results  are  influenced  by  the  age  of  the 
individuals  and  other  factors.  The  power  of  fusion  which 
comes  with  convergence  at  the  near  point  will  be  considered 
in  Section  8.  It  is  interesting  to  notice  that  although  there 
are  considerable  differences  of  opinion  as  to  the  fusion  power 
for  adduction  when  the  test  object  is  at  a  distance  of  six 
meters,  on  the  other  hand  the  observers  agree  quite  well  as 
to  the  power  of  abduction,  superduction,  and  subduction. 


302  Maximum  Fusion  Power 

Reference  will  be  made  to  this  point  also  in  connection  with 
pathological  aspects  of  the  question. 

§  6.  What  is  the  Amount  of  the  Maximum  Fusion 
Power  of  the  Various  Groups  of  Muscles  ? — It  is  impos- 
sible to  state  this  even  approximately,  for  the  reason  that 
when  we  use  prisms  of  gradually  increasing  strength,  so  as 
to  "  exercise  "  the  muscles,  the  result  depends  principally  on 
the  methods  employed  and  the  persistence  shown  by  the 
individual.  Tscherning  (B  230)  has  called  attention  to  the 
readiness  with  which  eyes  adjust  themselves  to  unusual 
positions  in  physiological  experiments,  and  has  shown  that 
it  is  easy  to  increase  the  power  of  adduction  within  a  few  days 
simply  by  systematic  practice  with  proper  prisms.  No  dis- 
comfort is  experienced  after  the  experiment,  and  for  all 
practical  purposes  the  eyes  are  the  same  in  spite  of  the 
increased  maximum  power. 

The  figures  given  by  Stevens  (B  725),  of  adduction  50°, 
as  well  as  the  context  of  his  statement,  indicate  that  he 
referred  to  the  maximum  power. 

Data  on  this  point  are  also  furnished  by  those  who  attempt 
to  "develop"  what  they  call "  latent  esophoria"  or  other  forms 
of  heterophoria.  This  will  be  considered  in  the  study  of  the 
clinical  aspect  of  the  subject.  Suffice  it  to  explain  here  that 
this  condition  has  been  compared  to  that  of  the  ciliary  muscle 
which  is  revealed  by  the  use  of  atropin.  In  the  latter  case 
we  have  a  certain  amount  of  hypermetropia  which  is  mani- 
fest, and  the  atropin  reveals  a  certain  additional  amount  which 
is  latent.  In  a  similar  way,  it  is  said  that  before  prisms  are  used 
a  certain  amount  of  muscular  power  is  "manifest,"  but  that 
with  the  prisms,  a  much  larger  amount  becomes  possible, 
and  hence  the  difference  is  "  latent."  In  proof  of  this, 
cases  are  cited  to  show  that  because  the  asthenopic 
symptoms  did  disappear  under  this  treatment,  therefore 
such  latent  heterophoria  must  have  existed,  and,  on  the 
other  hand,  when  relief  was  not  obtained,  then  this  latent 
heterophoria  did  not  exist,  even  though  the  muscles  were 
developed  to  a  very  marked  degree.  Without  commenting 
otherwise  on  this  proposition,  it  may  be  observed  that  this 
"  developing "  of  the  latent  heterophoria  is  something 


Balance  of  Groups  of  Muscles  303 

which  may  be  done  by  almost  any  one  who  has  normal  eyes, 
if  he  will  take  the  trouble  to  use  prisms  of  increasing 
strength  regularly  and  patiently,  even  for  a  comparatively 
short  time.  This  experiment  I  have  tried  at  different  times, 
and  the  fact  is  easily  demonstrated.  Such  experiments  on 
normal  or  abnormal  eyes  thus  show  how  readily  the  mus- 
cular power  can  be  increased,  and  indeed  no  limit  within 
the  bounds  of  credulity  seems  to  have  been  placed  on  the 
power  of  fusion.  In  a  word,  the  maximum  fusion  power  is  of 
some  interest  for  the  sake  of  comparison,  but  certainly  is 
not  of  as  much  importance  in  itself  as  many  writers  on  the 
subject  would  lead  us  to  believe. 

§  7.  Balance  of  Power  in  Groups  of  Muscles.— It  should 
not  be  inferred  that  relative  weakness  in  a  certain  group  of 
muscles  as  measured  in  terms  of  prisms,  is  an  indication  of 
inability  of  that  group  of  muscles  to  do  its  work  in  a  physio- 
logical manner.  In  different  individuals  whose  eyes  are 
practically  normal,  the  minimum  power  of  adduction,  for 
example,  may  be  quite  small,  occasionally  only  half  of  the 
average,  but  in  these  individuals  we  find  ordinarily  that  the 
minimum  power  of  the  opposing  group  is  also  less  than 
normal,  and  often  in  a  corresponding  degree. 

On  the  other  hand,  there  are  individuals  who  have  a  mini- 
mum power  of  adduction  largely  in  excess  of  the  average,  and 
in  these  we  are  apt  to  find  a  correspondingly  large  amount  of 
abductive  power.  In  other  words,  it  is  certain  that  in  the 
normal  condition  we  may  have  decided  variations  in  the 
relative  strength  of  different  individuals,  but  that  the  balance 
between  the  opposing  groups  of  muscles  remains  in  a  general 
way  about  the  same.  A  pair  of  scales  will  balance  whether 
there  is  a  weight  of  ten  grams  or  a  hundred  grams  on  either 
side. 

§  8.  Test  of  Muscle  Balance  with  Convergence. — We 
have  seen  that  when  the  visual  axesare  parallel,and  the  eyes  are 
in  the  apparent  static  condition  (as  ordinarily  tested),  hetero- 
phoria  is  about  as  frequent  as  orthophoria.  That  is  not  the 
case  when  accommodation  and  convergence  are  brought  into 
action.  This  is  an  important  fact.  In  order  to  appreciate 
its  significance,  let  us  understand  (a)  how  we  are  to  make  our 


304  Tests  of  Balance  with  Convergence 

measurement,    (£)    how   we   are     to   designate   the    results 
obtained,   and  (c)  what   these  results  really    mean.       - 

(a)  In  order  to  determine  whether  this  "balance" 
exists  at  the  working  distance,  the  ordinary  method 
is  to  use  a  vertical  line  with  a  dot  in  the  center,  such 
as  was  suggested  first  by  Graefe.  For  this  purpose 
a  short  thick  line,  with  a  small  dot  at  its  center  is 
often  employed  (Fig.  202).  But  this  is  confusing, 
and  unsatisfactory  replies  result.  It  is  there- 
fore better  to  use  a  long  fine  line  with  a  large 
distinct  dot  (Fig.  203),  and  if  this  is  drawn 
on  one  edge  of  the  paper  on  which  the  small 
test  letters  are  printed,  it  is  always  at  hand 
for  the  examiner.  The  usual  test  with  this, 
^  as  is  well  known,  is  to  place  before  one  eye 
of  the  subject  a  prism  with  the  base  down, 
and  ascertain  whether  the  vertical  line  and 
dot,  when  viewed  at  the  near  point,  appears 
directly  above  the  dot  seen  with  the  uncov- 
ered eye,  or  whether  the  upper  dot  is  above 
and  to  the  right,  or  above  and  to  the  left  of 
the  other  dot. 

FIG.  202.       (b)  If  the  line  is  a  long  one  and  the  prism 
Line  and  js  noj.  ^QQ  strong,  then,  when  the  individual 

Graefe     sces    on^v    one  ^on&   vertical  line  with  two 
As  it      dots  in    its    course,  we    say    that   "  muscle 
should  not  balance  "  exists  for  that  point,  whether  it  be 
be  drawn,  three,  four,  or  five  meter  angles  of  conver- 
gence.    This  condition  is  often  described  as  "  ortho- 
phoria  at  the  near   point."      That    term,    however, 
is  both  contradictory  and  indefinite.    If  orthophoria 
is  a   "  tending   of  the  visual   lines  in    parallelism," 
evidently    that   can    not   occur    with   convergence.    I.G'  2°3^ 
Moreover,  the  term  "  near  point "  alone  is  indefinite,    dot  Of 
and,  unless  otherwise  specified,  we  should  understand   Graefe. 
by  it  three  meter  angles  of  convergence.     Of  course    As  il 
when  the   measurement  is  made  in  this  way,  if  thej/* 

J  drawn. 

two  dots  are  not  in  the  same  vertical  line  they  can 

be  made  to  appear  so  by  placing  a  second  prism  at  right- 


Stereoscopes  305 

angles  to  the  first,  as  when  correcting  a  heterophoria.  The 
position  and  strength  of  this  second  prism  then  shows  the 
kind  and  degree  of  heterophoria  which  exists  with  conver- 
gence at  a  certain  number  of  meter  angles,  whatever  that 
may  be. 

(c)  Finally,  as  to  results  of  these  tests  in  normal  eyes. 
An  examination  of  the  non-asthenopic  eyes  of  the  103  per- 
sons just  referred  to  showed  that  perfect  muscle  balance  at 
the  near  point  of  three  meter  angles  was  present  in  a  fraction 
over  93  per  cent.  This  is  a  very  much  larger  percentage  than 
that  in  which  we  find  orthophoria  at  the  far  point.  More- 


/*-  _/ 

FIG.  204. — The  reflecting  stereoscope  of  Wheatstone. 

over,  the  greater  the  degree  of  accommodation  and  conver- 
gence, up  to  the  limit  of  a  comfortable  working  distance, 
the  more  constantly  do  we  find  perfect  muscle  balance. 

§  9.  Stereoscopes. — In  the  clinical  portions  of  this  study 
we  shall  find  that  the  stereoscope  is  of  decided  assistance  in 
treating  certain  forms  of  muscular  difficulties;  therefore,  aside 
from  its  intrinsic  interest  in  connection  with  convergence,  it 
is  advisable  at  this  point  to  understand  exactly  what  is  meant 
by  this  instrument.  For  we  must  distinguish  two  varieties, 
the  reflecting  and  the  refracting  stereoscope.  The  one  in- 
vented by  Wheatstone,  in  1848,  was  the  reflecting  stereoscope 
(Fig.  204).  In  this  the  picture  seen  by  the  right  eye,  fcr 
example,  is  reflected  into  a  mirror  (ab)  which  is  placed  at 
an  angle  of  forty-five  degrees  to  the  axis  of  vision.  If  this 


306 


Stereoscopes 


mirror  is  turned  laterally,  it  is  evident  that  the  eye  of  the 
observer  must  make  a  corresponding  amount  of  convergence 
or  divergence  in  order  to  be  in  line  with  the  reflected  ray.  In 
the  original  stereoscope  of  Wheatstone,  the  picture  and  the 
mirror  were  stationary.  In  later  modifications,  for  clinical  pur- 
poses the  two  mirrors  are  attached  by  a  hinge,  and  thus  the 
angle  at  which  they  stand  can  be  altered  as  desired. 

In  1849  Brewster  constructed  the  familiar  refracting  stereo- 
scope (Fig.  205).  In  this,  each  picture  is  seen  with  the  cor- 
responding eye  through  a  convex  lens 
so  decentered  as  to  act  as  a  strong 
prism.  This  causes  not  only  a  slight 
magnification  of  the  pictures,  but  also 
makes  them  overlap  slightly,  and  in 
doing  so  produces  a  single  retinal 
image. 

The  clinical  value  of  the  refracting 
stereoscope  is  indicated  by  the  number 
of  modifications  which  it  has  under- 
gone. Among  those  most  in  use  are 
the  stereoscopes  of  Bull,  Richard 

FIG.  205.-The  refracting  Derby  (Fig;  2°6)>  and  °thers  with   sim' 

stereoscope  of  Brewster.  ilar  adaptations.      As  these  all  depend 

on  the  same  physiological    principle, 

they  need  only  be  mentioned  here.  Their  use  will  be  referred 
to  under  the  treatment  of  certain  forms  of  heterophoria  and 
heterotropia. 

Most  of  the  refracting  stereoscopes  are  made  so  that  the 
object  is  viewed  only  with  considerable  convergence,  but 
long  ago  Noyes  extended  the  long  arm  of  the  instrument, 
which  carries  the  card  a  distance  of  three-fourths  of  a 
meter  or  more.  With  this  simple  modification  it  is  possible 
to  push  the  picture  as  far  away  as  it  is  possible  to  see  the 
details  with  exactness.1 

The  stereoscope  cards  already  in  use  are  so  varied  that  it 
would  seem  superfluous  to  call  attention  to  any  one  of  them, 


1  Pictures  especially    adapted   for  ophthalmologists    are    published   by  C. 
Eckenrath,  Berlin,  and  by  others  in  this  country. 


Stereoscopes 


307 


were  it  not  that  I  have  found  a  simple  arrangement  to  serve 
an  excellent  purpose,  not  only  for  physiological  experi- 
ments, but  for  clinical  purposes.  This  card  is  seen  in  Fig. 
207,  its  advantage  being  largely  in  its  simplicity.  Before 
one  eye  there  is  drawn  a  horizontal  line  with  a  circle  at  its 
center,  the  distance  from  the  center  of  the  circle  being  marked 
off  in  millimeters  from  zero  to  forty.  Before  the  other  eye  a 


FIG.  206. — Derby's  modification  of  the  Brewster  stereoscope. 

vertical  line  is  drawn.  At  its  center  there  is  a  circle  of  the 
same  size  as  the  circle  which  is  in  the  center  of  the  horizontal 
line,  and  from  the  center  of  this  circle  in  the  vertical  line 
twenty  millimeters  are  marked  off  in  each  direction.  The 
method  of  using  such  an  arrangement  suggests  itself  at  a 
glance.  When  the  stereoscope  is  placed  in  front  of  the  ob- 
server, if  he  has  good  binocular  vision  it  is  easy  for  him,  by 
adjusting  the  distance,  to  make  the  two  circles  overlap.  If 


308  Stereoscopes 

he  then  approaches  the  card  perhaps  only  a  short  distance, 
the  circles  separate,  or,  if  the  tendency  to  fusion  is  great,  the 
card  can  be  brought  comparatively  close  to  the  eyes  before 
the  circles  are  separated.  If  the  bar  on  which  the  rack  for 
the  card  slides  is  marked  off  in  centimeters,  it  is  evidently 
easy  to  note  the  distance  from  the  eyes  at  which  the  circles 


•2 
FIG.  207. — Stereoscopic  picture  adapted  for  lateral  or  vertical  deviations. 

begin  to  slip  away  from  each  other.  By  keeping  a  record 
of  this  distance  and  of  the  number  of  millimeters,  right  or 
left,  up  or  down,  which  the  two  circles  separate,  or  tend  to 
separate  from  the  other,  we  have  a  method  of  measuring  the 
behavior  of  the  ocular  muscles  under  the  same  conditions  at 
different  times.  Even  the  patient  himself  can  thus  observe 
what  changes,  if  any,  take  place  in  his  condition. 


DIVISION  III. 


Relation  of  Accommodation  to  Convergence. 
Relative  Accommodation. 

§     i.     Definition. — The  amount  of  accommodation  which 
it  is  possible  for  an  individual  to  exert  or  relax  with  relation  to 
a  given     degree    of    convergence    is 
called  the  relative  accommodation. 

In  Figure  208  let  us  suppose  the 
eyes  to  be  accommodated  and  also 
converged  to  the  point  p.  Then  if 
concave  glasses  of  gradually  increas- 
ing strength  are  placed  before  the 
eyes,  the  person,  while  retaining  the 
same  degree  of  convergence,  will  be 
able  to  increase  his  accommodation 
up  to  a  certain  limit.  This  degree 
would  be  represented  by  the  strong- 
est concave  glasses  which  he  can 
overcome,  and  is  equivalent  to  ac- 
commodation to  a  point  nearer  than  that  to  which  the 
eyes  were  converged,  for  example,  to  pa.  The  distance  ppa 
will  then  be  the  positive  part  of  the  relative  accommodation. 

In  like  manner,  if  the  person  continues  to  converge  to 
the  point  p,  and  convex  glasses  of  gradually  increasing 
strength  are  placed  in  succession  before  his  eyes,  he  .will  be 
able  to  relax  his  accommodation  up  to  a  certain  limit — for 
example,  to  p,.  The  distance  pp,  will  then  be  the  negative 
part  of  the  relative  accommodation.  Evidently,  the  total 
range  of  relative  accommodation  is  equal  to  the  sum  of  the 
negative  and  positive  portions,  or  to  the  distance  p/p,,. 

§  2.  Illustration  of  the  Ranges  of  Accommodation 
for  Varying  Degrees  of  Convergence. — A  clearer  idea  of 

309 


FIG.  208.— Illustration  of 
relative  accommodation. 


310        Definition  of  Relative  Accommodation 


the  relative  accommodation  can  be  obtained  if  we  repre- 
sent graphically,  even  though  ap- 
proximately, the  ranges  which  are 
found  with  varying  degrees  of  con- 
vergence. 

For  this  purpose,  let  us  suppose 
that  we  are  testing  the  eyes  of  a 
young  emmetrope,  and  that  the  fol- 
lowing figures  represent  the  ranges 
of  accommodation  which  have  been 
found  by  placing  in  succession,  first 
concave  and  then  convex  glasses  be- 
fore the  eyes. 

Thus,  in  o  of  Figure  209,  we  see 
what  occurs  when  this  individual 
looks  at  test  letters  six  meters  dis- 
tant. Since  the  visual  axes  are  then 
parallel  we  cannot  speak  of  accom- 
modation in  relation  to  convergence. 
But  let  us  place  concave  glasses  of 
-3  or  -3.25  before  the  eyes  of  our 
emmetrope.  He  may  not  be  able 
to  read  the  letters  at  first,  but  after 
a  few  seconds  and  after  an  effort,  of 
which  he  is  usually  conscious,  he  can 
see  the  letters.  Evidently  his  crys- 
talline lenses  have  become  sufficient- 
ly convex  to  overcome  the  concave 
glasses  in  front  of  the  eyes.  That  is, 
he  has  exerted  3  or  3.25  diopters  of 
positive  relative  accommodation.  If 
FIG.  sog.-Diagram  show-  WQ  attempt  to  represent  this  in  a 

ing  the  range  of  accommoda- 

tion  with  varying  degrees  of  diagram,  we  must  draw  two  parallel 
convergence.  The  amount  lines  corresponding  to  the  positions 
of  accommodation  which  is  of  the  visual  axes,  and  indicate  on 

possible  with  each  meter  angle  each  ljne  ft        jnt  tQ  whkh  each  js 

of   convergence  is  shown   by 

then    accommodated.     As    for    the 


the  vertical  line. 

negativeportion  of  hisrelative  accom- 

modation, there  is  none  to  represent.     For  if  convex  glasses 


Illustration  of  Relative  Accommodation      311 

are  placed  before  the  eyes  of  an  emmetrope  he  has  no  accom- 
modation to  relax,  and  the  image  falling  in  front  of  the 
retina,  the  print  becomes  blurred.  In  other  words,  when 
an  emmetrope  looks  with  parallel  visual  axes,  it  is  evidently 
possible  to  exert  only  the  positive  part  of  the  relative 
accommodation. 

Let  us  consider  next  the  amount  of  relative  accommoda- 
tion which  is  possible  with  a  slight  convergence, — for  instance, 
with  suitable  test  types  placed  one  meter  distant.  If 
we  place  a  —  2.5  or  —3.  before  each  eye  he  will  or- 
dinarily, after  a  few  seconds,  see  the  print  distinctly.  In 
other  words,  the  positive  portion  of  the  relative  accommo- 
dation is  2.5  or  3  diopters  as  the  case  may  be.  Or  again,  by 
placing  a  plus  0.5  or  plus  0.75  before  each  eye  he  will,  in  a 
similar  way,  relax  the  accommodation  to  that  degree — that 
is,  the  negative  portion  of  the  relative  accommodation  is  0.5 
or  0.75.  It  is  possible  to  represent  approximately  this 
amount  of  positive  and  of  negative  accommodation  exerted 
with  relation  to  that  degree  of  convergence.  Thus  in  figure 
209,  if  the  vertical  line  representing  the  relative  accommoda- 
tion be  drawn  to  a  scale,  the  total  relative  range  for  one 
meter  angle  is  shown  as  in  that  part  (i)  of  the  diagram.  With 
convergence  to  one-third  of  a  meter,  there  is  a  little  less  of 
the  positive  portion  and  more  of  the  negative,  and  this  can 
be  represented  in  a  similar  manner.  With  convergence  to 
one-fourth,  one-fifth,  one-sixth,  and  one-seventh  we  notice  in 
succession  that  the  positive  portion  of  the  relative  accommo- 
dation constantly  decreases,  while  the  negative  portion  con- 
stantly increases.  This  increase  or  decrease  is  not  always 
regular,  but  there  is  always  a  tendency  to  that  regularity. 
With  convergence  at  one-eighth  of  a  meter  the  emmetrope 
can  not  overcome  any  concave  glass  at  all ;  in  other  words, 
there  is  no  positive  portion,  but  it  requires  all  the  effort 
which  the  ciliary  muscles  can  make  to  accommodate  enough 
to  see  the  test  object.  But  the  accommodation  can  be 
relaxed  to  a  degree  which  is  represented  by  glasses  of 
plus  5.  diopters.  Suppose  the  convergence  is  increased  still 
further,  to  a  point  one-ninth  of  a  meter  in  front  of  the  eyes. 
Our  emmetrope  may  converge  to  that  point,  but  it  is  impos- 


312        Illustration  of  Relative  Accommodation 

sible  for  him  to  accommodate  to  any  object  which  is  so  near. 
Hence  convex  glasses  must  be  used.  Let  us  suppose  plus 
1.75  to  be  sufficient.  With  their  aid,  the  image  can  again  be 
brought  upon  the  retina,  and  the  weakest  convex  glasses 
which  will  enable  him  to  see  distinctly  with  that  degree  of 
convergence  represent,  of  course,  the  nearest  point  of  relative 
accommodation.  Now  if  slightly  stronger  convex  glasses  are 
placed  in  front  of  the  eyes,  the  individual  simply  relaxes  his 
effort  at  accommodation  in  proportion.  If  other  convex 
glasses,  a  little  stronger,  are  placed  before  the  eyes,  he  relaxes 
his  accommodation  more,  and  still  a  little  more,  all  the  while 
maintaining  that  same  degree  of  convergence  at  one-ninth  of 
a  meter.  If  we  continue  placing  in  turn  stronger  convex 
glasses  before  the  eyes,  we  find  a  point  where  our  emmetrope 
can  no  longer  relax  his  accommodation,  and  the  glasses  then 
used  —  for  example,  plus  6.25  —  represent  the  farthest  limit 
of  the  negative  portion  of  the  relative  accommodation.  Thus, 
with  that  degree  of  convergence,  although  the  relative  accom- 
modation is  all  negative,  there  still  exists  a  considerable 
range,  represented  in  this  case  by  the  difference  between  plus 
1.75  and  plus  6.25.  Strictly  speaking,  allowance  should  also 
be  made  for  the  distance  of  the  glasses  from  the  eyes,  but 
this  will  be  considered  later.  Suppose  the  convergence  to  be 
increased  still  farther — for  example,  to  a  point  at  one-tenth  of 
a  meter.  Here  again  the  accommodation  can  not  be  exerted 
to  a  point  so  very  near,  and  the  test  letters  suitable  for  that 
distance  become  even  more  indistinct  than  in  the  last 
instance.  Moreover,  greater  assistance  would  be  needed  for 
the  accommodation  in  order  to  bring  the  image  on  the  retina. 
That  is,  instead  of  using  a  plus  1.75  before  each  eye  a  plus  4 
or  plus  4.5  would  be  required.  Thus  again,  the  weakest 
glasses  necessary  to  supplement  the  crystalline  lens  would 
represent  approximately  the  nearest  point  of  the  negative 
portion  of  the  relative  accommodation,  while  the  strongest 
glasses — plus  7,  for  example — which  give  distinct  vision  would 
represent  the  greatest  amount  of  relaxation.  In  other  words, 
with  the  visual  axes  crossing  at  one-tenth  of  a  meter,  the 
relative  range  of  accommodation  again  would  be  all  negative 
and  the  range  would  be  a  little  shorter  than  in  the  last 


Illustration  of  Relative  Accommodation       313 

instance.  With  convergence  to  one-eleventh  of  a  meter,  the 
weakest  convex  glass  necessary  being  5.5,  for  example,  and 
the  strongest  one  possible  being  only  7.25,  the  difference 
between  those  two  is  less — that  is,  the  range  of  accommo- 
dation is  shorter.  With  convergence  of  one-twelfth  of  a 
meter,  we  would  find  the  range  still  all  negative  and  still 
shorter.  At  one-thirteenth  of  a  meter  we  find  that  glasses 
as  strong  as  plus  8  diopters  are  necessary  to  enable  our 
emmetrope  to  see  the  proper  line  of  letters  at  that  dis- 
tance. But  we  also  find  by  trying  other  glasses  that  8  plus 
is  the  strongest  glass  with  which  he  can  see — that  is,  he  can- 
not relax  the  accommodation  any  farther.  In  other  words, 
there  is  no  range  of  negative  relative  accommodation  with 
that  degree  of  convergence,  and  consequently  the  range  must 
be  represented  graphically  not  by  a  line  but  by  a  single 
point. 

§  3.  Desiderata  for  the  Accurate  Measurement  of  Rela- 
tive Accommodation. — The  foregoing  gives  only  an  idea  of 
what  relative  accommodation  is,  and  how  it  can  thus,  in  a 
simple  way,  be  represented  graphically  in  a  given  case.  It 
is  essential,  however,  that  we  learn  how  these  measurements 
can  be  made  accurately,  not  only  because  of  what  they 
teach  the  physiologist,  but  because  of  the  decidedly  im- 
portant data  which  they  often  furnish  to  the  clinician.  For 
this  purpose  it  is  necessary  to  have  certain  conveniences  and 
appliances,  and  it  is  desirable  to  become  acquainted  with 
these  before  attempting  to  make  an  examination  even  of  a 
pair  of  normal  eyes.  These  desiderata  are : 

(A)  A  visuometer  with   which    to   measure   the  inter- 
ocular  base  line.     This  has  already  been  described. 

(B)  A  table  to  show  the  actual  size  of  the  meter  angle 
with  various  degrees  of  convergence.     This  has  also  been 
given. 

(C}  An  optometer  (as  seen  in  Fig.  210)  or  instrument  for 
measuring  relative  accommodation  and  convergence.  In  an- 
other section  an  account  will  be  given  of  other  appliances 
which  have  been  used  by  different  observers  for  making  these 
measurements.  At  present  it  is  best  to  describe  the 
latest  model  of  the  instrument  used  in  these  tests,  with 


3 1 4      Optometer  for  Relative  Accommodation 

the   secondary  appliances   for  this  purpose,  because  it   is 
the  simplest  and  seems  to  be  the  most  practical  form. 

A  square  brass  rod,  A  A',  about  105  centimeters  long,  forms 
the  stem  of  a  "  T  "  of  which  the  cross-bar  is  another  piece 
eight  or  ten  centimeters  long.  The  longer  bar  is  supported 
at  either  end  on  a  foot-piece,  F  and  F'.  The  upright  is  so 
arranged  that  the  height  of  the  bar  from  the  table  can  be 
varied  some  twenty  or  thirty  centimeters.  On  either  side  of 
the  short  bar  there  is  a  disc  or  "  carrier  "  of  brass,  D  D,  fifty 
millimeters  wide  and  three  or  four  millimeters  thick,  and 


A' 


FIG.  210. — Optometer  of  the  author. 

each  disc  has  near  its  center  a  slot  in  the  form  of  an 
arc,  graduated  in  degrees,  C  and  C'.  The  center  of  this  arc 
is  exactly  thirty  millimeters  beyond  the  end  of  the  long 
bar.  The  zero  point  on  the  disc  marks  the  position  of  an 
imaginary  line  parallel  with  the  long  arm  of  the  T — that  is, 
the  position  when  the  parallel  visual  axes  are  directed  at  an 
object  in  the  horizontal  plane.  The  degrees  on  the  arcs 
measure  therefore  any  convergence  of  the  axes.  Each  slot 
is  provided  with  a  small  nut,  N  and  N',  accurately  fitted  and 
carrying  an  index  line  which  marks  its  place  with  reference 
to  the  degree  on  the  arc  of  the  carrier.  Each  nut  is  also 


Optometer  for  Relative  Accommodation      315 

provided  with  a  short  vertical  bar  supporting  an  arc  in 
which  a  lens  can  be  placed.  The  nut  can  be  tightened  or 
loosened  by  means  of  a  thumb  screw,  whenever  it  is  desired 
to  change  the  angle  at  which  the  spherical  glass  is  set  with 
reference  to  the  axes  of  the  eyes  of  the  observer.  The  dis- 
tance between  the  two  discs  or  carriers  can  be  adapted  to  any 
pair  of  eyes,  by  means  of  a  screw  of  double  rotation,  S  S', 
which  brings  them  together  or  separates  them  as  desired. 
This  distance  between  the  center  of  a  given  pair  of  eyes  is 
recorded  on  an  index  which  passes  along  a  linear  slot  in 
the  carrier  and  over  a  millimeter  scale. 

The  long  bar  of  the  T  is  graduated  in  meter  angles  and 
carries  a  four-sided  revolving  frame  (O)  which  can  be  slid 
back  and  forth.  On  each  one  of  the  four  sides  a  small  card  of 
test  objects  is  placed.  The  plane  of  the  side  which  is  turned 
toward  the  observer  marks  the  distance  in  meter  angles  from 
the  interocular  base  line  of  the  eyes  under  observation. 
That  distance  can  be  easily  made  to  correspond  with  one, 
two,  three,  and  four  or  more  meter  angles.  The  use  of  this 
whole  arrangement  for  measuring  relative  accommodation 
will  be  shown  later. 

Next  let  us  consider  more  exactly  the  test  types  used 
with  this  instrument. 

In  former  measurements  made  of  relative  accommodation, 
one  of  the  difficulties  encountered  was  to  obtain  test  objects 
of  the  proper  size  in  proportion  to  the  distance  at  which  they 
were  viewed.  The  nearest  approach  to  accuracy  heretofore 
was  obtained  probably  by  Bisinger  (B  778), who  made  use  of  the 
minute  dots  advised  by  Bourchard.  A  few  trials  with  these 
showed  their  imperfections.  They  are  not  sufficiently  numer- 
ous or  varied,  and  are  therefore  easily  learned  by  the  patient, 
and  even  the  smallest  are  too  large  when  viewed  through  the 
strong  convex  glasses  used  at  the  higher  meter  angles.  More- 
over, when  these  are  magnified  their  edges  appear  blurred. 

Fortunately,  however,  it  is  possible  by  the  aid  of  photog- 
raphy to  overcome  these  difficulties  to  a  considerable  de- 
gree. In  any  set  of  types  properly  constructed  on  the  basis 
of  the  angle  of  minimum  vision,  we  know  that  each  element 
of  the  letter  subtends  an  angle  of  fifty-five  seconds.  If 


FIG.  2ii.- 


316     Test  Types  for  Relative  Accommodation 

the  letter  to  be  seen  by  the  normal  eye  at  100  meters 
measures  133  millimeters  in  both  heighth  and  breadth,  then 
the  one  intended  for  one  meter  would  be  one  one-hundredth 
of  that  size.  For  our  purpose,  therefore,  it  is  necessary 
first  to  obtain  a  series  of  types  so  constructed  as  to  be 
visible  at  the  following  distances  in  meters:  100-50-33-25- 
20-16.6 — 14.2 — 12.5 — 1 1.1-10-9.0 — 8.  3 — 7.6 —  7.1 — 6.6 — 6.2 
—5.8—5.5—5.2—5. 

When  these  are  reduced  on  the  glass   plate  of  a   camera 
to  one  one-hundredth  of  their  original  size,  they  are  adapted 
for  testing  the  vision  at  each  meter  angle  respectively,  from 
one  to  twenty.  Fig.  211. 

Theoretically  this  is  simple  enough,  but  there 
•  BB          are  practical  difficulties  not  only  in  having  such 
a  series  properly  made,  but  especially  in  obtain- 
ing  a  clear  picture  of  proper  size   on   a   pure 
white  background. 

Nothing  concerning  this  could  be  found  in  the 
ters  for  test    literature,  and  after  having  several  sets  photo- 
ing  relative     graphed,  I  learned  that  Javal  had   devoted  con- 
accommoda-    siderable  time  and  care  to  preparing   a   similar 
turn.          series  of  microscopic  letters.     With  the  sugges- 
tions obtained  from  him,  a  series  was   secured 

tually  used  are 

photographed  which  is  perhaps  as  accurate  as  can  be  produced 
and  are  there-  by  our  present  methods.  But  with  these  tests, 
fore  much  as  wjth  all  others,  difficulties  are  presented  when 
glasses  are  placed  at  a  short  distance  before  the 
eye.  Particles  of  dust  or  moisture  on  the  lens,  or  slight 
decentering,  easily  blur  the  vision.  Also  when  these  tests 
are  made  on  persons  whose  vision  is  not  perfect  or  cannot  be 
brought  near  to  the  standard  of  perfection  by  suitable  glasses, 
a  corresponding  allowance  must  be  made  in  the  results  ob- 
tained. Fortunately,  however,  these  errors  are  usually  not 
great,  and  by  making  corrections  of  the  ametropia  when 
necessary,  the  types  described  serve  all  practical  purposes 
very  well.  They  are  mounted  on  one  side  of  the  small  square 
frame  which  slides  along  the  long  bar.  One  or  two  series  of 
such  letters  can  be  placed  on  other  faces  of  the  same  square 
frame  if  greater  exactness  is  desired. 


Recording  of  Relative  Accommodation       3 1 7 

(D)  Another  desideratum  for  the  measurement  of  rela- 
tive accommodation  is  a  proper  blank  on  which  the  data  can 
be  conveniently  recorded.  Such  a  blank  should  indicate  by 
proper  headings  the  date,  case  number,  name,  residence, 
and  age  of  the  person,  with  the  length  of  the  interocular 
base  line. 

The  first  column  shows  the  distance  in  meter  angles  of 
the  object  from  the  individual  when  a  given  test  is  made. 

The  second  gives  the  amount  of  accommodation  which  the 
individual  ordinarily  does  exert  with  a  given  degree  of  con- 
vergence, this  being  dependent  on  the  kind  and  degree  of 
his  ametropia,  if  any  exists. 

In  the  third  column  is  entered  the  strongest  concave 
glasses  which  he  can  overcome  at  each  meter  angle  of  con- 
vergence. 

In  the  fourth  is  recorded  the  actual  strength  of  the  diop- 
tric system  with  these  glasses,  after  correction  has  been 
made  for  their  distance  from  the  eye,  or  in  other  words  what 
is  really  the  positive  part  of  the  relative  accommodation. 

The  fifth  column  shows  the  strongest  convex  glasses  which 
can  be  overcome,  or  when  all  of  the  accommodation  is  nega- 
tive, the  strongest  and  also  the  weakest  glasses  which  the 
individual  can  overcome  at  each  meter  angle  of  convergence. 

In  the  sixth  is  entered  the  actual  strength  of  these 
glasses,  or  of  the  two  pairs  of  convex  glasses  just  mentioned, 
after  correction  has  been  made  for  their  distance  from  the 
eyes,  or  in  other  words,  what  is  the  actual  negative  part  of 
the  relative  accommodation. 

Finally,  in  the  last  column  the  total  range  of  the  relative 
accommodation  is  entered.  This  of  course  is  the  sum  of  the 
positive  and  the  negative  amounts,  or  when  all  the  relative 
accommodation  is  negative,  the  total  range  is  then  shown  by 
the  difference  between  the  strongest  and  the  weakest  convex 
glasses,  they  being  corrected  for  the  distance  from  the  eyes. 

One  of  the  blanks  with  the  data  inserted  is  shown  on 
page  319. 

§  4.  Details  of  the  Method  of  Measuring  Relative 
Accommodation  with  the  Optometer. — An  accurate  meas- 
urement of  relative  accommodation  with  all  possible  degrees 


318    Measurement  of  Relative  Accommodation 

of  convergence  is,  without  question,  rather  a  tedious  proced- 
ure demanding  no  small  amount  of  exactness  and  patience 
on  the  part  of  the  examiner  and  of  the  one  examined.  In 
order  however  to  understand  the  process  and  the  important 
relation  of  accommodation  to  convergence  from  the  clinical 
standpoint,  it  is  necessary  to  trace  our  way,  for  once  at  least, 
through  the  details  of  the  measurement  of  a  pair  of  normal 
eyes.  Fortunately  we  shall  find  later,  when  we  come  to 
consider  the  clinical  value  of  these  measurements,  that  the 
tests  which  are  most  important  can  be  made  easily  and  quite 
quickly.  It  is  essential  for  such  an  observation  that  the 
person  examined  should  have  an  average  degree  of  in- 
telligence, and  eyes  of  about  the  same  amount  of  vision, 
so  that  the  tendency  to  fusion  of  the  images  is  always 
maintained. 

As  a  preliminary  step,  the  interocular  base  line  should  be 
measured  by  means  of  the  visuometer  (page  215).  Let  us 
suppose  that  to  be  58  millimeters.  The  subject  or  patient 
is  then  seated  in  a  comfortable  position  at  the  end  of  a  table 
long  enough  to  hold  the  optometer.  The  horizontal  arm  of 
the  instrument  is  raised  or  lowered  till  the  glasses  are  on 
the  same  level  as  the  eyes  of  the  patient,  the  nut  in  the  arc  in 
each  slot  is  brought  to  the  zero  point,  and  by  means  of  the 
thumb  screw  on  the  side  the  two  carriers  are  also  separated 
from  or  made  to  approach  each  other  until  the  distance  be- 
tween the  centers  of  the  glasses,  as  indicated  by  the  scale  in 
front  of  the  carriers,  registers  58  millimeters.  In  other  words, 
the  instrument  is  so  arranged  that  when  the  eyes  of  the 
person  are  in  the  primary  position  he  can  look  straight 
through  the  glasses  at  an  object  six  meters  distant  on  the 
wall  in  front.  It  should  be  stated  that  the  measurement  of 
relative  accommodation  with  parallel  axes  can  be  made 
almost  as  well  without  any  special  apparatus,  and  before 
the  subject  is  seated  in  front  of  the  optometer.  Then  he 
simply  looks  at  the  usual  test  types  through  glasses  which 
are  held  in  the  ordinary  frame. 

However  that  part  may  be  done,  it  is  convenient  to  have 
at  hand  the  table  showing  the  meter  angles  with  different 
degrees  of  convergence,  and  the  blank  referred  to  above. 


Measurement  of  Relative  Accommodation    319 

Whether  the  optometer  or  a  spectacle  frame  is  used,  at  a 
distance  of  six  meters  in  front  of  the  patient  there  should  be 
hung  the  series  of  ordinary  test  types  properly  illuminated. 
The  examiner  can  manipulate  the  glasses  with  his  left  hand 
and  make  notes  with  his  right.  These  numerous  details 
being  completed,  we  are  ready  to  commence  the  examina- 
tion. Let  us  suppose  that  the  subject  is  an  emmetrope. 
Of  course  there  is  no  convergence  when  one  reads  the 
test  types  six  meters  distant,  so  we  write  zero  in  the 


Date. 
Name. 
Residence. 
Right  Eye  V  = 
Left  Eye     V  = 


Relative  Accommodation.     Case  No. 

Age.         Base  Line. 


£•§> 

Accom. 

—  Glass 
Overcome 

Actual 
+  Accom. 

+  Glass 
Overcome 

Actual 
—  Accom. 

Total  of 
Relative. 

o 

0 

—  3.25 

2-95 

o 

o 

2-95 

I 

i 

—  3 

2-59 

-i-    0.75 

0.72 

3-31 

2 

2 

—  3 

2.44 

H-    i-5 

1-38 

3.82 

3 

3 

—  2.5 

1-95 

+     2. 

1.74 

3-69 

4 

4 

—    2 

1.46 

+    2 

25 

1.85 

3-31 

5 

5 

—  1.5 

i.  06 

+    2 

5 

1.90 

2.96 

6 

6 

—  i 

.71 

+   3 

25 

2.34 

3-05 

7 

7 

—  0.75 

•51 

+   4 

5 

3.08 

3-59 

8 

8 

0 

O 

+    5 

5 

3.61 

3.61 

9 

9 

o 

O 

6.25 

i-75 

3-85 

•94 

2.91 

10 

10 

0 

0 

7 

4-5 

4.02 

1.81 

2.21 

ii 

ii 

o 

o 

7-25 

5-5 

3-65 

2.60 

1.05 

12 

12 

0 

o 

7-5 

7-55 

346 

3.38 

.08 

13 

13 

o 

0 

8 

8 

3-27 

3-27 

.O 

H 

15 

16 

17 

18 

first  space  of  this  column.  Over  the  next  column  in 
the  table  we  write  "Accommodation."  The  first  entry 
which  we  make  in  this  column  must  also  be  zero,  for  if 


320     Measurement  of  Relative  Accommodation 

the  emmetrope  reads  the  test  line  marked  six,  it  is  evident 
that  accommodation  is  entirely  at  rest. 

Next,  let  us  ascertain  what  is  the  positive  part  of  the  rela- 
tive accommodation  when  the  patient  is  looking  at  the  letters 
six  meters  distant.  For  this  purpose  minus  glasses  are  placed 
in  succession  before  the  eyes,  commencing  with  the  weaker 
and  advancing  to  the  stronger,  till  we  find  the  strongest 
which  do  not  distinctly  blur  the  line  which  should  be  seen  at 
that  distance.  These  glasses  represent  approximately  the 
degree  of  extra  accommodation  made  by  the  ciliary  muscles. 
Let  us  suppose  that  they  are  minus  3.25.  This  should  be 
recorded.  Accordingly  over  the  third  column  we  write 
"  minus  glass  overcome  "  and  in  the  first  place  in  that  column 
we  enter  —  3.25.  It  has  been  said  that  this  glass  represents 
approximately  the  amount  of  accommodation  exerted  by  the 
crystalline  lens.  It  does  not  represent  that  accommodation 
exactly,  however,  for  the  reason  that  instead  of  being  a  part 
of  the  crystalline  lens  and  having  the  same  nodal  points,  it  is 
in  reality  situated  at  a  certain  distance  in  front  of  the  eye. 
That  is,  the  arc  of  the  slot  in  the  carrier  is  at  a  distance  of 
0.03  from  the  nodal  point  of  the  eye.  It  is  therefore  neces- 
sary to  ascertain  what  is  the  real  amount  of  the  accom- 
modation exerted,  when  a  minus  3.25  glass  is  placed  three 
hundredths  of  a  meter  in  front  of  the  nodal  point. 

In  the  expression  -p -,  =  -^-f +  -FT  (Chap.  II.,  Sec. 

3),  we  must  remember  that  with  parallel  visual  axes  P  =  <x>, 
and  also  here,  as  later,  the  plus  and  minus  signs  for  F  give 
a  minus. 
Th  I  I        _  £ 

Hence      -^  =  0=^-^-3.25 

P' O.O3    =    O.3O8 

P'  =  0.338  meters. 

Or   expressing  this  distance  as  a  reciprocal  to  show  the 
lens  value, 
I 


0.338 


=  2.95 


Measurement  of  Relative  Accommodation    321 

This  gives  us  2.95  diopters  as  the  real  amount  of  the  positive 
part  of  the  relative  accommodation  exerted  when  the  minus 
3.25  glasses  are  in  front  of  the  eyes.  That  should  be  recorded. 
Therefore  at  the  top  of  the  next  column  in  the  prepared  table 
we  write  "Actual  positive  accommodation  "  and  in  the  first 
space  in  this  column  we  enter  2.95. 

The  next  step  is  to  measure  the  negative  portion  of  the 
relative  accommodation  by  bringing  convex  glasses  before 
the  eyes.  But  it  happens  that  the  person  we  have  selected 
has  normal  eyes.  He  therefore  cannot  relax  the  accommo- 
dation any  farther  when  looking  at  an  object  with  parallel 
visual  axes,  and  any  convex  glass,  however  weak,  blurs  the 
types.  But  that  fact  should  also  be  recorded.  Consequently, 
over  the  next  column  in  the  table  we  write  "  Convex  glass 
overcome  "  and  enter  zero  in  the  first  place  in  that  column. 
Over  the  next  column  we  write  "  actual  negative  accom- 
modation "  and  in  the  first  place  in  that  column  also  enter 
zero. 

Next  let  us  take  convergence  at  one  meter  angle.  This 
time  the  subject  looks  not  at  the  test  types  six  meters 
distant,  but  at  the  uppermost  of  those  already  described 
on  the  optometer  at  a  distance  of  one  meter.  Then,  remem- 
bering that  the  distance  between  the  eyes  of  the  individual 
under  examination  is  fifty-eight  millimeters,  we  consult  the 
table  giving  the  angles  of  convergence  and  find  that  with  a 
base  line  of  fifty-eight  millimeters,  when  the  visual  axes 
cross  at  one  meter,  there  is  a  convergence  of  each  of 
one  degree  and  thirty-nine  minutes.  Accordingly,  we 
loosen  the  thumb  screw  under  the  arc  of  one  carrier, 
push  the  nut  with  the  arc  inward  until  the  index 
marks  about  one  degree  and  a  half,  and  there  make 
the  nut  fast.  The  same  is  done  with  the  nut  bearing 
the  arc  for  the  glass  before  the  other  eye.  The  centers  of 
the  two  glasses  are  now  in  the  line  of  the  visual  axes 
when  the  individual  under  examination  is  looking  at  the 
test  type,  one  meter  distant.  We  record  as  we  proceed. 
As  there  is  a  convergence  of  the  visual  axes  of  one  meter 
angle,  in  the  second  space .  of  the  first  column  we  write 
one.  As  an  emmetrope  at  that  distance  exerts  naturally 


322    Measurement  of  Relative  Accommodation 

an  accommodation  of  one  diopter,  in  the  second  space 
of  the  second  column  also  we  write  one.  We  next  find 
the  strongest  concave  glass  that  can  be  overcome  in  the  man- 
ner before  described.  Let  us  suppose  that  to  be  minus  3. 
This  is  written  in  the  second  space  of  the  third  column. 
That  glass  gives  only  the  approximate  amount  of  positive 
relative  accommodation.  It  should  be  borne  in  mind  that 
the  object  of  the  test  is  to  ascertain  the  real  amount  of  posi- 
tive relative  accommodation,  and  therefore  we  make  use  of 
our  same  formula,  remembering  that  the  distance  P'  is  a 
fractional  part  of  the  meter,  and  have 

h  3-=  4-03 


P'-  .03       i 

7 


P'  =     —  +  .03  =.278 
4-03 


That  is,  with  this  lens  (  —  3.)  before  each  eye,  the  distance 
P'  extends  0.278  meters  from  K.  Expressing  this  in  diopters 
(as  the  reciprocal) 


But  the  emmetropic  eye  without  a  glass,  converged  at 
one  meter,  of  itself  exerts  one  diopter  of  accommodation. 
So  that  the  real  positive  part  of  the  relative  accommodation 
at  one  meter  is  not  3.59  diopters  but 

3.59  —  i.  =  2.59  diopters. 

This  is  therefore  entered  in  the  second  space  on  the  fourth 
column. 

Next  we  search  the  negative  part  of  the  relative  accom- 
modation with  this  degree  of  convergence.  For  that  pur- 
pose convex  glasses  are  tried  in  succession  before  the  eyes, 
the  strongest  which  will  allow  the  test  types  at  the  distance 
of  one  meter  to  be  seen  distinctly  indicating  approximately 
the  negative  portion  of  the  relative  accommodation  at  that 
point.  Let  us  suppose  them  to  be  -j-  0.75.  In  order  to 


Measurement  of  Relative  Accommodation    323 

ascertain  the  actual  amount  of  the  negative  relative  accom- 
modation at  that  point  we  return  to  our  formula  (i)  and  sub- 
stituting we  have 

—  -.75=  -28 


.03       i 

7  -.03 


That  is,  with  the  lens  0.75  before  the  eye,  the  distance  P' 
extends  3.6  meters  from  K.  Expressing  this  in  diopters  (as 
the  reciprocal) 

i 

—  >  =  0.277 
3«6 

But  as  before,  the  emmetropic  eye  without  a  glass,  when 
converged  at  one  meter,  naturally  exerts  one  diopter  of 
accommodation  ;  so  the  real  negative  part  of  the  relative 
accommodation  at  one  meter  is  I.  —  .277  =  0.72  diopter. 
This  is  recorded  on  the  second  line  of  the  sixth  column. 
The  negative  and  positive  portions  added  together  give 
us  the  total  range  of  the  relative  accommodation  with  the 
visual  axes  crossing  one  meter  distant,  and  that  amount  — 
namely,  3.31  —  is  entered  in  the  second  place  of  the  last 
column. 

Theoretically,  measurements  should  be  made  successively 
with  a  convergence  of  the  visual  axes  at  two,  three,  or  four 
meter  angles,  and  at  each  step  the  proper  entry  made  in  the 
chart.  Practically,  it  is  better  to  vary  this  routine  in  several 
details.  But  to  avoid  confusion  in  this  description  it  is 
assumed  that  each  observation  at  the  different  meter  angles 
is  made  one  after  the  other,  and  recorded  in  succession. 

We  notice  that  the  positive  part  of  the  relative  accommo- 
dation grows  gradually  less  while  the  negative  part  gradu- 
ally increases.  At  last  we  reach  a  point  where  the  crystalline 
lens  cannot  overcome  any  minus*  glass  whatever.  This, 
therefore,  is  the  limit  of  the  positive  portion  of  the  relative 
accommodation.  In  the  case  of  our  emmetrope,  this  occurs 
with  a  convergence  of  eight  meter  angles,  hence  in  the  third 


324     Measurement  of  Relative  Accommodation 

column  opposite  that  point  we  write  zero  and  in  the  fourth 
column  zero  also.  Then,  testing  with  the  glasses  shows  an 
ability  to  overcome  plus  5.5,  and  making  the  correction  for 
this  according  to  the  formula,  we  find  3.61  as  the  actual 
negative  relative  accommodation,  or  in  this  case  the  total 
relative  accommodation  is  3.61. 

Beyond  this  point  we  have  to  deal  only  with  the  negative 
portion  of  the  relative  accommodation,  but  when  we  test  this 
exactly,  we  find  there  is  a  certain  range  of  plus  glasses  which 
our  emmetrope  can  overcome.  For  example,  when  the  test 
types  are  placed  at  this  distance  it  is  possible  to  relax  the 
accommodation  to  such  a  degree  as  to  see  clearly  with  plus 
0.75  and  also  in  succession  with  plus  I,  2,  3,  and  so  on  up  to 
plus  6.5.  Evidently,  therefore,  there  is  a  range  of  vision  be- 
tween these  two  points,  and  this  is  the  range  of  the  negative 
portion  of  the  relative  accommodation.  These  tests  are  made 
with  convex  glasses  in  a  manner  exactly  similar  to  the  tests 
already  described,  and  in  each  case  the  correction  showing 
the  actual  negative  portion  of  the  accommodation  is  calcu- 
lated by  the  same  formula  which  we  have  already  used.  The 
only  difference  in  making  this  portion  of  the  test  is  that  in 
order  to  continue  the  record  systematically  it  is  necessary 
to  subdivide  the  fifth  and  sixth  columns  below  the  point 
where  the  positive  portion  of  relative  accommodation  has 
ceased  to  exist.  We  therefore  draw  a  line  down  the  center 
of  the  fifth  and  sixth  columns.  On  the  right  side  of  this 
dividing  line  in  the  fifth  column  we  write  plus  1.75,  and 
on  the  left  side  we  write  plus  6.6.  Also,  having  made  the 
correction  for  each  of  these  according  to  the  formula, 
and  obtained  for  each  the  negative  portion  of  the  relative 
accommodation,  we  write  the  results  respectively  on  the 
left  and  the  right  side  of  the  line  which  divides  the  lower 
portion  of  the  sixth  column.  Then  we  find  the  total 
range  of  accommodation  by  subtracting  the  lesser  from  the 
larger  numbers  in  each  horizontal  line  in  the  sixth  column, 
and  place  the  remainder  in  the  seventh  column. 

The  measurements  are  carried  on  in  the  same  way  with  a 
convergence  of  nine  meter  angles,  ten  meter  angles,  etc.,  un- 
til at  last  we  find  a  point  where  there  is  no  range  between 


How  to  Plot  Relative  Accommodation       325 

the  two' con  vex  glasses  which  can  be  placed  before  the  eye. 
In  other  words,  there  is  no  further  range  in  the  relative 
accommodation. 

§  5.  How  to  Plot  Relative  Accommodation.— Bon- 
ders found  it  convenient  to  represent  relative  accommodation 
and  convergence  in  the  form  of  curves  or  lines  on  the  ordinary 
system  of  co-ordinates.  In  these,  the  horizontal  line  is  divided 
into  equal  parts,  each  one  of  which  from  left  to  right  repre- 
sents one  meter  angle  of  convergence,  from  zero  to  ab'out 
sixteen  or  eighteen.  At  each  of  these  points  of  division  a  per- 
pendicular is  erected.  The  latter  are  also  divided  into  equal 
parts,  each  of  which,  from  below  upward,  represents  one  di- 
opter of  accommodation  from  zero  to  about  sixteen  or  eigh- 
teen. At  each  of  these  points  of  division  a  horizontal  line  is 
drawn.  These  diagrams  were  elaborated  by  Bisinger  ( B  778  ) 
in  Nagel's  laboratory  and  are  copied  in  many  of  the  familiar 
text-books.  At  present,  only  a  word  concerning  them  is 
necessary.  Thus  if  with  one  diopter  of  accommodation  there 
is  found  to  be  one  meter  angle  of  convergence,  we  would  in- 
dicate that  by  placing  a  dot  at  the  junction  of  the  first  hori- 
zontal with  the  first  vertical  lines;  if  with  two  diopters  of 
accommodation  there  are  found  to  be  also  two  meter  angles 
of  accommodation,  we  would  indicate  that  by  placing  a  dot  at 
the  junction  of  the  second  horizontal  with  the  second  verti- 
cal line  to  the  end.  In  other  words,  our  diagonal  from  the 
lower  left-hand  to  the  upper  right-hand  corner  of  the  series 
of  squares  would  represent  both  accommodation  and  conver- 
gence (Fig.  212).  But  if  the  positive  part  of  the  relative  ac- 
commodation is  recorded  above  the  diagonal  and  the  nega- 
tive part  below  it,  it  is  easy  to  see  at  once  how  a  series  of 
measurements  is  entered.  These  details  which  are  appar- 
ently so  self-evident  are  repeated,  for  the  reason  that  such 
diagrams,  at  least  in  outline,  ought  to  form  a  part  of  the 
clinical  records  in  a  considerable  portion  of  our  cases  of 
anomalous  action  of  the  ocular  muscles. 

After  the  measurements  are  made  and  the  results  properly 
entered  in  the  table,  we  can  then  construct  our  curves.  Let 
us  suppose  that  we  wish  to  represent  graphically  the  relative 


326       How  to  Plot  Relative  Accommodation 

accommodation  of  the  emmetrope  whom  we  have  examined. 
We  found  with  parallel  visual  axes  that  it  was  possible  to 
exert  an  accommodation  of  2.95.  Therefore,  commencing  at 
the  point  marked  zero  on  the  squares,  we  count  upward  three 
squares,  and  just  below  that  we  place  a  mark.  (Fig.  213.) 
From  the  sixth  column  of  our  table  we  find  that  the  negative 
portion  of  the  relative  acccommodation  is  zero,  consequently 


13 

12 

II 

10 

9 

8 

7 

6 

5 

4 

3 

2 

I 


O     1    2    34    5    6    7    8    9    IO  11    12  13  14 

FIG.  212. — Diagrammatic  representation  of  accommodation  and 
convergence  when  they  are  equal. 


the  negative  portion  would  begin  at  the  zero  point  in  the 
squares. 

Next,  with  a  convergence  of  one  meter  angle,  we  find  the 
positive  portion  of  the  relative  accommodation  to  be  2.59. 
Therefore,  beginning  at  the  point  where  the  diagonal  crosses 
the  first  horizontal  line  we  count  upward,  and  place  a  dot  on 
the  vertical  line  about  half-way  between  the  second  and  third 
squares  above  that  point.  With  this  same  degree  of  con- 
vergence we  find  also  the  negative  part  of  the  relative  accom- 
modation is  0.72.  Therefore,  from  the  same  point  where  the 


How  to  Plot  Relative  Accommodation        327 

diagonal  cuts  the  first  vertical  line  we  measure  down  about 
three-fourths  of  the  first  square,  and  place  a  dot  there. 

The  same  process  is  followed  of  counting  from  the  diag- 
onal upward  in  squares  or  fractions  of  a  square  for  the  posi- 
tive part  of  the  relative  accommodation,  and  downward  in 
squares  or  fractions  of  a  square  for  the  negative  part. 
When  there  is  no  longer  any  positive  portion  of  the  relative 
accommodation,  the  curve  crosses  the  diagonal.  Beyond 
that  point  there  is  only  a  range  in  the  negative  portion  of 


14 
13 
12 
11 
1O 


3 


/ 


O  •  1    2    34-    5   6    7   8    9   10  11    12  13  14 

FIG.  213. — Lines  showing  the  relative  accommodation 
as  plotted  in  a  given  case. 

the  relative  accommodation.  For  both  of  these,  we  count 
down  from  the  diagonal  in  squares  or  fractions  of  a  square, 
that  perpendicular  line  being  chosen,  of  course,  which  indi- 
cates the  proper  degree  of  convergence.  Finally,  we  come 
to  a  point  where  there  is  no  longer  any  range  even  in  the 
negative  accommodation,  and  here  there  is  but  one  dot  or 
mark  to  make.  Thus  we  find  that  we  have  the  lines  of  the 
squares  marked  with  a  series  of  points,  and  it  is  only  neces- 
sary to  connect  these  points  by  a  line  in  order  to  have  before 


328        How  to  Plot  Relative  Accommodation 

us  the  curves  desired.  The  plotting  of  the  curves  of  the  case 
just  measured  is  shown  in  Figure  213. 

When  the  individual  under  examination  is  an  emmetrope, 
of  course  the  second  column  of  the  table  corresponds  with 
the  first — that  is  to  say,  at  a  distance  of  one  meter  the 
emmetrope  exerts  one  diopter  of  accommodation,  at  two 
meters  he  exerts  two  diopters,  at  three,  three  diopters, 
etc.  If,  however,  the  patient  is  ametropic,  that  does  not 
hold  good.  Thus,  if  we  have  to  deal  with  a  myopia  of  four 
diopters,  such  a  person  converges  in  the  usual  way,  but  does 
not  exert  any  accommodation  until  the  object  which  is  fixed 
approaches  nearer  than  four  meter  angles — that  is,  with  a 
convergence  of  five  meter  angles  there  would  be  an  accom- 
modation of  only  one  diopter.  Then  with  a  convergence 
of  six  meter  angles  there  would  be  an  accommodation  of 
two  diopters  ;  with  seven,  of  three  diopters,  etc. 

When  we  wish  to  represent  the  relative  convergence  of  such 
a  myope  by  a  curve,  it  is  necessary  that  the  diagonal  should 
begin,  not  at  the  lower  left-hand  corner  marked  zero,  but  on 
the  vertical  line  four  squares  below  that,  and  extend  diagonally 
upward  and  to  the  right  as  did  the  diagonal  for  emmetropia. 

On  the  other  hand,  a  hypermetrope  has  to  make  a  certain 
amount  of  accommodation  even  for  the  distance.  Therefore  if 
we  have  to  deal  with  a  hypermetrope  of  two  diopters,  such 
a  person  with  a  convergence  of  one  meter  angle  would  evi- 
dently exert  an  accommodation  of  three  diopters;  with 
convergence  of  two  meter  angles  there  would  be  an  accom- 
modation of  four  diopters,  etc.  To  represent  the  relative 
accommodation  by  means  of  a  curve  in  such  a  case,  the 
diagonal  would  not  begin  as  in  the  case  of  emmetropia,  but 
on  the  vertical  line  two  squares  above  that  point,  and  be 
extended  diagonally  and  to  the  right  in  the  same  manner  as 
in  emmetropia.  In  either  case  the  curve  is  constructed  from 
the  data  in  the  table  according  to  the  same  plan  we  have 
followed  when  dealing  with  emmetropia. 

§  6.  Other  Methods  of  Measuring  Relative  Accom- 
modation.— It  will  be  seen  from  the  foregoing  description 
that  the  general  plan  of  these  measurements  is  similar  to 
that  adopted  by  Bonders  and  elaborated  by  Bisinger  (B  778) 


Other  Methods  of  Measurement  329 

and  Nagel  (B  780).     But  it  should  be  understood  that  it  is 
not  the  only  method  nor  is  it  the  most  accurate. 

Another  method  of  measuring  relative  accommodation 
was  suggested  by  Pereles  (B  787).  The  principle  in- 
volved is  shown  in  Fig.  214.  L  P,  L  :  PI  are  the  test  objects, 
5  and  Si  mirrors,  and  A  and  A  i  represent  the  position  of 
the  eyes.  As  the  objects  are  moved  in  a  curve  toward  the 
points  7/i,  the  mirrors  5  and  S  1}  which  are  attached  to  the 
objects,  turn  outward,  necessitating  convergence  just  as  in 
the  reflecting  stereoscope.  The  amount  of  convergence  is 
therefore  shown  at  once  by  the  degrees  on  the  arc  through 
which  the  object  turns. 


(D'0 


FIG.  214. — Pereles'  arrangement  for  measuring  relative 
accommodation  and  convergence. 

Acting  on  this  suggestion  of  Pereles',  I  had  an  apparatus 
constructed  which  was  essentially  the  same,  except  as  to  a 
simplified  and  improved  form  of  the  object  looked  at.  This 
arrangement  is  seen  in  Fig.  215.  Probably  the  most  exact 
instrument  for  this  purpose  is  the  one  used  by  Hess  of 
Wiirzburg,  and  special  acknowledgment  should  be  made  at 
this  point  for  the  many  suggestions  obtained  from  him 
bearing  on  this  part  of  the  study.  The  principle  in- 
volved in  his  apparatus  is  similar  to  the  one  suggested  by 
Pereles,  but  differs  from  it  in  that  the  object  to  be  ob- 
served is  a  point  or  two  points  of  light  as  in  the  Schreiner 
test  for  accommodation.  Moreover,  instead  of  mirrors  to 
mark  the  degree  of  convergence  which  the  eyes  make,  in 
order  to  see  the  reflection  of  these  points  Hess  used  the 


330 


Other  Methods  of  Measurement 


reflecting  surface  of  a  prism  before  each  eye.  The  general 
arrangement  of  this  apparatus  is  seen  in  Fig.  216.  After 
reading  his  description,  one  of  the  instruments  was  ordered 
from  the  same  maker,  but  the  measurements  with  it  were 
not  satisfactory  and  later,  after  seeing  the  working  of  the 


FIG.  215. — Sketch  of  Pereles*  apparatus  (as  modified  by  the  author) 
for  measuring  relative  accommodation. 

original  instrument  at  the  clinic  in  Wurzburg,  they  were 
repeated.  The  difficulty  is  that  the  person  who  uses  it 
must  be  well  trained  in  laboratory  work,  as  the  results  de- 
pend largely  upon  his  ability  to  keep  his  eyes  in  the  proper 
position  (not  an  easy  task),  and  also  to  determine  exactly 
whether  two  points  of  light  or  one  are  visible,  as  in  the 
ordinary  Schreiner  experiment.  In  a  word,  while  this 


FIG.  216. — Arrangement  of  Hess  for  measuring  relative  accommo- 
dation and  convergence. 

method  is  doubtless  more  exact  than  the  others,  it  is  not 
well  adapted  to  clinical  purposes. 

Let  us  examine  still  further  the  apparent  differences  in  the 
range  of  relative  accommodation  as  measured  by  different 
methods.  It  should  be  understood  that,  although  the  earlier 
method  suggested  by  Bonders  is  still  the  simplest,  and  for 


Other  Methods  of  Measurement  331 

practical  purposes  the  most  convenient,  on  the  other  hand  a 
source  of  error  is  undoubtedly  present  with  tests  made  in  that 
way.  The  fact  is  that  the  limits  of  the  range  of  accommo- 
dation are  not  invariable.  At  one  moment  the  person  under 
examination  may  give  one  reply,  and  at  another  moment  a 
reply  a  little  different,  depending  upon  the  variation  in  the 
amount  of  the  accommodation  which  he  is  able  to  exert  at 
that  instant,  in  his  efforts  to  see  the  small  letters  more  dis- 
tinctly. For  this  reason  the  curves  as  given  by  Donders 
himself  are  somewhat  inexact,  principally  because  the  test 
types  used  were  imperfect.  By  employment  of  the  photo- 
graphed test  types  which  have  been  here  described,  this  error 
is  very  much  lessened.  But  even  with  these,  the  tests  show 
that  instead  of  representing  the  limits  of  the  positive  and 
negative  part  of  relative  accommodation  by  means  of  a 
curve,  the  fact  is  that,  in  an  exact  sense,  these  limits  of  the 
positive  and  negative  parts  of  relative  range  can  be  repre- 
sented by  straight  lines.  This  has  been  dwelt  upon  at  con- 
siderable length  by  Hess  in  his  masterly  contribution  to  this 
subject,  which  forms  a  part  of  the  second  edition  of  Graefe- 
Saemisch.  The  two  lines  which  he  gives  as  representing  the 
positive  and  the  negative  parts  of  the  relative  accommoda- 
tion are  drawn  by  him  as  parallel  to  the  diagonal. 

From  the  foregoing  it  may  be  inferred  that  when  we 
attempt  to  measure  the  amount  of  relative  accommoda- 
tion in  any  given  case,  the  results  depend  to  a  considerable 
extent  upon  the  method  employed. 

Thus  if  the  subject  is  tested  by  looking  through  spherical 
lenses  at  a  test  object,  which  is  practically  of  the  same  size 
for  different  degrees  of  accommodation,  then,  when  we  plot 
the  results  we  obtain  a  well  marked  curve  to  represent  the 
positive  part  of  the  relative  accommodation,  the  convexity 
of  this  curve  being  directed  upward,  while  the  negative  part 
of  the  accommodation  is  also  represented  by  another  curved 
line  whose  convexity  is  directed  downward.  Or,  again,  if  we 
still  use  spherical  glasses,  but  make  the  test  object  of  care- 
fully prepared  photographic  letters,  as  here  described,  then, 
when  the  results  are  plotted  we  obtain  lines  with  less  curve  to 
represent  the  positive  and  the  negative  accommodation. 


332 


Other  Methods  of  Measurement 


And  finally,  if  we  discard  the  spherical  glasses,  depending  on 
the  observation  of  a  trained  experimenter,  and  use  only  a  spot 
of  light  for  the  test  object,  as  in  the  instrument  of  Hess,  we 
obtain  a  more  or  less  straight  line,  parallel  to  the  diagonal, 
which  represents  the  relative  convergence  (Fig.  217). 

This  apparent  difference  is  evidently  dependent  upon  the 


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FIG.  217. — Straight  lines  showing  the 
range  of  relative  accommodation 
(Hess). 

fact  that  when  we  test  the  positive  and  negative  part  of  the 
relative  accommodation  by  placing  concave  and  convex  lenses 
before  the  eye,  no  matter  how  accurately  graduated  the  ob- 
ject which  is  looked  at,  the  person  under  examination  does 
vary  somewhat  this  relative  amount  of  the  accommodation, 
while  with  the  instrument  of  Pereles  such  variation  is 
lessened,  and  with  that  of  Hess  it  is  still  less. 

But  the  practical  fact  is,  that  the  concave  and  convex 
spherical  glasses  furnish  us  at  once  with  a  very  simple  method 
by  which  these  tests  can  be  made.  For  that  reason  they  are 
to  be  preferred  for  clinical  purposes.  Practically,  therefore, 
it  makes  but  little  difference  whether  we  obtain  as  the  result 
of  our  measurements  a  more  or  less  curved  line  or  a  straight 
one  to  represent  the  relative  accommodation.  Indeed,  it  must 
be  kept  in  mind  that  the  real  object  of  this  long  inquiry  into 


Other  Methods  of  Measurement  333 

relative  accommodation  is  not  to  determine  any  small  theo- 
retical question  as  to  the  value  of  methods  or  the  degree  of 
accuracy  obtained  with  them.  The  question  resolves  itself 
into  this  :  Is  the  positive  part  of  the  relative  accommodation 
large  or  is  it  small  with  regard  to  the  amount  of  convergence 
demanded  by  that  individual  for  his  usual  work  ?  If  the  posi- 
tive part  is  large,  then,  other  things  being  equal,  that  person 
has  comfort ;  if  it  is  not  large,  or  if  glasses  will  not  make  it  so 
for  him,  then,  other  things  being  equal,  he  has  discomfort. 
If  the  older  methods  of  testing  the  relative  accommodation 
with  spherical  glasses  are  sufficiently  exact  to  define  that  con- 
dition, for  practical  purposes,  then  these  methods  should  be 
preferred  because  they  are  simple,  rather  than  because  they 
are  absolutely  exact.  The  important  bearing  of  this  physio- 
logical fact  will  appear  clearly  when  we  come  to  the  study  of 
insufficient  accommodation. 

Finally  we  may  ask,  is  this  relation  of  accommodation  to 
convergence  exactly  the  same  for  all  normal  eyes?  No.  Af- 
ter making  the  measurements  and  plotting  the  curves  for  even 
a  few  persons,  it  becomes  apparent  that  this  depends  upon  the 
age  or  other  factors,  and  that  there  are  individual  peculiarities 
modifying  the  form  which  the  curve  assumes.  Indeed,  this 
may  differ  somewhat  for  the  same  person  at  different  times. 
How  is  it  possible  then  to  say  whether  the  curve  obtained  for 
a  certain  individual  at  a  certain  time  is  entirely  normal?  It 
is  impossible,  just  as  it  is  impossible  to  say  whether  a  certain 
individual  at  a  certain  time  is  or  is  not  in  "  perfect  health." 
But  one  condition  is  no  more  indefinite  than  the  other. 
In  spite  of  all  these  slight  variations  the  data  obtained  by 
measurement  of  the  relative  accommodation  are  perfectly 
reliable  for  clinical  purposes. 

§  7.  How  the  Range  of  Relative  Accommodation  with 
Parallel  Visual  Axes  is  Influenced  by  Age. — The  emme- 
trope  whom  we  selected  for  measurement  of  the  relative 
accommodation  was  still  in  early  life,  and  we  have  found  that 
he  had  V=  £  and  with  minus  3.25  diopter  glasses  he  could 
read  test  type  at  six  meters  quite  as  well  as  without  any 
glasses.  Now  if  we  were  to  examine  a  large  number  of  em- 
metropic  persons  under  twenty  or  twenty-five,  each  one  would 


334  Relative  Accommodation  Influenced  by  Age 

overcome  glasses  of  about  that  strength.  In  childhood,  say 
from  ten  to  fifteen,  there  is  not  infrequently  an  ability  to 
overcome  minus  three-and-a-half  or  occasionally  a  minus  four, 
but  more  positive  relative  accommodation  is  exceptional. 
On  the  other  hand,  when  we  examine  older  persons,  those 
over  forty-five  or  fifty  and  beyond,  then  we  find  that  this 
power  of  relative  accommodation  with  parallel  visual  axes 


1 

13 
12 

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FIG.  218. — Curve  showing  the  amount  of  positive  part  of 
the  relative  accommodation  at  different  ages  when  the  visual 
axes  are  parallel. 

gradually  decreases.  On  examining  my  notes  of  the  amount 
of  relative  accommodation  in  young  persons,  especially  in 
soldiers,  and  also  the  records  of  examinations  which  have 
been  made  of  patients  and  others,  it  appears  that  a  curve 
could  be  constructed  which  represents  the  gradual  decrease 
with  advancing  age  of  the  positive  part  of  the  relative  accom- 
modation with  parallel  visual  axes.  This  curve  is  seen  in  Fig. 
2 1 8.  It  shows  that  at  ten  or  fifteen  the  positive  part  of  the 


Relative  Accommodation  Influenced  by  Age  335 

relative  accommodation  may  even  exceed  three  diopters.  From 
about  fifteen  to  twenty  or  twenty-five  it  does  not  vary  far 
from  that  amount,  and  at  about  thirty  or  thirty-five  it  begins 
to  lessen.  By  forty  it  is  hardly  more  than  two.  Another 
decade  finds  the  power  still  less,  so  that  by  fifty  or  sixty  or 
sixty-five  the  ciliary  muscle  has  lost  entirely  its  power  of 
overcoming  a  minus  glass  when  viewing  the  distant  object. 


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FIG.  219. — Representation  of  the  change  in  the  position  of  the  far 
and  of  the  near  point  with  advancing  age  as  given  by  Hess,  together 
with  the  curve  found  by  the  author  to  represent  the  decrease  in  the 
amount  of  relative  accommodation  with  parallel  axes,  also  with 
advancing  age. 

This  curve  seems  to  be  of  importance  because  it  represents 
what  takes  place  not  only  in  emmetropia  but  in  ametropia. 
For,  after  a  refractive  error  is  corrected,  if  the  ciliary  muscle 
is  still  in  normal  condition,  we  find  that  with  parallel  axes 
this  positive  part  of  the  relative  accommodation  remains 
practically  the  same.  That  is,  in  early  life,  if  there  is  a  hyper- 
metropia  of  one  diopter,  then  such  a  person  can  overcome 


336      Clinical  Measurement  of  Accommodation 

only  a  minus  2  or  minus  2.5,  etc.  In  order  to  show  just 
how  this  curve  compares  with  the  curve  for  the  change  of  the 
position  of  the  near  and  the  far  point  as  first  given  by  Don- 
ders,  or  as  it  is  more  recently  elaborated  by  Hess,  I 
have  copied  the  illustration  given  by  the  latter,  and  insert- 
ed also,  this  third  curve  which  represents  the  changes  in 
the  positive  part  of  the  relative  accommodation  with 
parallel  visual  axes.  The  three  curves  together  are  seen  in 
Fig.  219. 

§  8.  How  is  Relative  Accommodation  Measured  for  Clin- 
ical Purposes  ? — While  the  method  which  has  been  detailed 
for  measuring  relative  accommodation  shows  what  the  plan  is, 
and  how  it  is  possible  to  measure  every  part  of  a  curve  if  we 
desire  to  do  so,  that  procedure  is  evidently  too  technical  and 
laborious  to  form  any  considerable  part  of  an  ordinary  clinical 
examination.  Although  we  are  dealing  now  only  with  the 
physiological  aspects  of  our  subject,  we  may  ask  ourselves  at 
this  point  how  we  can  obtain  some  idea  concerning  the  rela- 
tive accommodation  in  a  pair  of  eyes  in  the  least  possible 
time.  It  is  not  difficult.  Indeed,  most  practitioners  are 
accustomed  to  include  an  outline  of  this  measurement  in 
their  routine  work.  For  the  superficial  examinations  with 
which  we  must  be  satisfied  at  trft  first  visit  of  a  patient,  it  is 
unnecessary  to  measure  the  base  line  or  to  attempt  any  cor- 
rection for  the  distance  of  the  glass  in  front  of  the  eye. 
Suppose  with  the  patient  before  us  we  have  ascertained  that 
he  can  read  print  6.  and  0.3  at  the  proper  distances;  natur- 
ally we  wish  to  ascertain  if  there  is  any  hypermetropia,  and 
in  doing  so  we  use  convex  glasses  in  gradually  increasing 
strength.  If  he  does  not  accept  any  of  these,  we  know  that 
with  parallel  axes  there  is  no  negative  part  of  the  relative 
accommodation.  If  he  does  accept  a  convex  glass,  that 
shows  that  he  is  a  hypermetrope  and  also  the  amount  which 
is  manifest,  and  that  gives  us  evidently  at  the  same  time 
the  negative  part  of  the  relative  accommodation.  Then  we 
place  minus  glasses  before  his  eyes  —  about  minus  3  if  he  is 
an  emmetrope,  or  if  he  is  an  ametrope  varying  their  strength 
proportionately.  This  gives  us  at  once  the  positive  part  of 


Clinical  Measurement  of  Accommodation     337 

the  relative  accommodation  with  parallel  visual  axes,  or  the 
sum  of  these  two  is  the  total  range. 

When  testing  the  relative  accommodation  with  conver- 
gence so  as  to  .obtain  only  the  first  general  idea  of  the 
conditions  present,  it  is  unnecessary  to  use  the  optometer,  or 
to  measure  exactly  the  degree  of  this  relative  accommoda- 
tion with  one  or  two  meter  angles  of  convergence.  It  is 
sufficient  to  determine  at  once  the  amount  of  relative  accom- 
modation with  convergence  at  three  meter  angles,  namely, 
at  about  the  reading  distance.  For  that  purpose  the 
test  types  which  the  normal  eye  can  see  at  one-third  of 
a  meter  are  given  to  the  patient,  and  having  made  sure 
that  the  distance  at  which  they  are  held  is  about  thirty- 
three  centimeters,  we  place  before  each  eye  the  strongest 
concave  glass  with  which  the  patient  can  still  read  that 
print  readily.  This  gives  the  positive  part  of,  the  relative 
accommodation,  understanding  that  we  must  subtract  in 
that  case  three  from  the  strength  of  the  glass,  because  con- 
vergence at  one-third  of  a  meter  itself  necessitates  for  the 
normal  eye  an  accommodation  of  three  diopters. 

In  a  similar  way,  by  placing  convex  glasses  before  the 
eyes  while  the  print  is  still  held  at  one-third  of  a  meter,  and 
by  ascertaining  what  is  the  strongest  convex  glass  with  which 
that  print  is  seen,  and  again  making  a  corresponding  cor- 
rection, we  have  the  negative  part  of  the  relative  accommo- 
dation with  convergence  at  three  meter  angles. 

Many  clinicians  are  accustomed  to  make  these  rough  tests 
of  relative  accommodation,  and,  as  already  stated,  they  often 
form  a  part  of  the  routine  examination  at  the  first  visit.  In 
simple  cases  where  one  examination  is  sufficient,  it  is  evi- 
dently a  waste  of  time  and  energy  to  attempt  any  such 
tedious  measurements  of  the  relative  accommodation  as 
have  been  described  here.  But  every  practitioner  knows 
how  certain  cases  persist  in  returning.  Moreover,  the  rough 
tests  which  we  make  at  the  first  visit  often  show  unmis- 
takably that  the  difficulty  depends  on  some  anomaly  of  the 
accommodation.  In  such  cases  it  is  our  evident  duty  to 
make  use  of  every  available  means,  to  obtain  all  the  data 
that  we  can  concerning  the  accommodation,  and  in  doing 


338       Importance  of  Relative  Accommodation 

this,  much  of  the  plan  for  measuring  the  relative  accommo- 
dation of  the  physiological  condition  is  useful  also  for 
clinical  purposes. 

§  9.  Clinical  Importance  of  Relative  Accommodation. — 

Attention  was  long  ago  called  to  the  important  relation 
which  exists  between  the  positive  and  negative  part  of 
the  relative  accommodation.  Donders  stated  this  in  italics 
(B  260,  page  114),  saying  that  "the  accommodation  can  be 
maintained  only  for  a  distance  at  which>  in  reference  to  the 
negative  part,  the  positive  part  of  the  relative  range  of  the 
accommodation  is  tolerably  great."  This  opinion  has  been 
corroborated  by  every  student  of  ophthalmology  during  the 
last  half  century.  Writers  have  expressed  the  important 
fact  in  terms  more  or  less  technical  and  clinicians  of  all 
grades  have  recognized  it,  usually  without  knowing  it. 
It  is,  to  refraction  work,  what  the  pole-star  is  to  the 
mariner. 

As  it  is  essential  to  normal  eyes  to  retain  a  "  tolerably 
great  "  positive  part  of  the  relative  accommodation,  so  is  it  in 
hypermetropia  to  keep  a  certain  part  of  that  in  reserve  as 
latent  instead  of  using  the  total  amount.  Every  time 
one  prescribes  a  glass  for  presbyopia  he  recognizes  this 
fact  whether  he  thinks  of  it  or  not,  for  as  the  ciliary  muscles 
gradually  lose  their  power,  the  very  object  of  the  convex 
glasses  is  to  keep  the  positive  part  of  the  relative  accommoda- 
tion thus  "  tolerably  great."  This  important  principle  has 
an  additional  significance  when  we  consider  it  not  alone 
with  reference  to  accommodation  but  in  the  relation  of 
that  act  to  convergence  also.  What  it  means,  practically, 
is  that  a  considerable  excess  of  power  of  accommodation, 
such  as  we  shall  find  in  spasm  of  the  ciliary  muscle,  and  in 
what  corresponds  to  that,  as  regards  convergence — namely, 
esophoria — may  exist  without  producing  much  discomfort, 
and  is  really  of  comparatively  little  clinical  importance.  On 
the  other  hand,  we  find  that  when  the  positive  part  of  the 
relative  accommodation  is  abnormally  lessened,  or  in  what 
corresponds  to  that  as  regards  convergence — namely,  exo- 
phoria — that  condition  is  much  more  apt  to  give  rise  to 


Importance  of  Relative  Accommodation      339 

asthenopic  symptoms  of  various  kinds.    They  will  claim  our 
attention  later. 

In    our  studies    of    the    pathological   conditions   of  the 
muscles  we  shall  find  the  most  important  and  apparently  the 
most  frequent  anomalies  are  those  which  involve  the  ciliary 
muscle.      Therefore  even  in  routine  examinations,  and  at 
the  first  visit,   it  is   desirable    to    determine   whether  the 
action  of  that  muscle  is  normal  or  excessive,  or  insufficient. 
At  least  a  general  idea  as  to  this  power  of  the  ciliary  muscle 
is  shown,  as  already  stated,  simply  by  placing  thus  a  minus  3 
glass  before  each  eye  and  asking  the  patient  to  read  again  the 
distant  test  type.     I  have  learned  to  regard  this  as  one  of  our 
most  important  tests.     For  if,  after  the  ciliary  muscles  have 
had   a  minute  or  two  in  which  to  adjust  themselves,  the 
person  can  still  read  as  well  as  before,  then  we  know  at  once, 
at  least  in  a  general  way,  that  there  is  no  imperfection  in 
the  power  of  the  ciliary  muscle,  apart  from  convergence. 
If  the  person  cannot    overcome    these    or    weaker   minus 
glasses  in  proportion  to   his   age  or   in   proportion  to  his 
ametropia,  then  we  at  once  suspect  some  insufficient  power 
of  the  ciliary  muscles.     Even  when  such  insufficient  accom 
modation  does  exist,  there  may  be  little  or  no  discomfort  at 
near  work,  especially  if  the  extraocular  muscles  are  excep- 
tionally strong  or  the  general  condition  or  the  occupation  of 
the  individual   unusually  favorable.     But  ordinarily,  if  the 
positive  part  of  the  relative  accommodation  is  insufficient 
with  parallel  axes,  and  also  with  convergence  at  one-third  of 
a  meter,  and  if  discomfort  and  headache  do  exist,  then  that 
clue  should  be  followed  up.     The  examinations  should  be 
repeated,  at   first  roughly,  if  desired,  with  convergence  at 
one-half  or  one-quarter  of  a  meter.     But  if  this  evidence 
points  in  the  same  direction,  and  if  the  discomfort  continues 
even    when   other  possible    causes    of    the    difficulty    are 
eliminated,  then   it   is    usually  worth   while   to   make   the 
data  complete  by  measuring  the  base  line  and  going  through, 
at  least  the  essential  parts  of  the  examinations  indicated. 
Of  late  years  American  ophthalmologists  particularly  have 
taken  great  pains  to  determine  the  condition  of  the  extra- 
ocular  muscles  and  have  been  so  engrossed  with  these  alone, 


340       Importance  of  Relative  Accommodation 

that  physiological  facts  concerning  the  intraocular  muscles 
which  were  demonstrated  long  ago,  and  which  still  are  of  the 
first  importance  clinically  have  been  forgotten.  It  is  well, 
therefore,  to  establish  on  a  firm  foundation  that  part  of  our 
clinical  work,  even  though  it  necessitates  this  long  and 
rather  wearisome  discussion  of  relative  accommodation. 


DIVISION  IV. 


Relation  of  Convergence  to  Accommodation. 
Relative  Convergence. 

§    i.     Definition. —  The  amount  of  convergence  which  it 


x' A' 


n 


nA 


Fm.  220. — Diagrammatic  representa- 
tion of  relative  convergence  with  paral- 
lel axes  and  at  three  different  degrees  of 
accommodation. 


is  possible  for  an  individual 
to  exert  or  relax  with  rela- 
tion to  a  given  degree  of  ac- 
commodation is  called  the 
relative  convergence.  In  Fig. 
220  let  us  suppose  the  eyes 
to  be  converged  and  also  ac- 
commodated to  the  point 
B.  Then  if  adductive  prisms 
of  gradually  increasing 
strength  be  placed  before 
the  eyes,  the  person,  while 
retaining  the  same  degree 
of  accommodation,  will  be 
able  to  increase  his  conver- 
gence up  to  a  certain  limit. 
This  degree  is  represented 
by  the  strongest  prisms, 
with  the  bases  outward, 
which  he  can  overcome, 
and  is  the  equivalent  of 
converging  to  a  point  nearer 
than  that  to  which  the  eyes 
are  accommodated. 

If  we  wish  to   represent 
graphically   this    point     of 
nearer  convergence, we  place 
it  somewhere   on  the    line 
The  distance  Ba  then   represents 


MB — for  example,  at  a. 

the  positive  part  of  the  relative  convergence. 

341 


342  Desiderata  for  its  Measurement 

Again,  let  us  suppose  that  while  the  person  still  accommo- 
dates for  the  point  B,  abductive  prisms  of  gradually  increasing 
strength  are  placed  before  his  eyes ;  he  will  then  be  able  to 
relax  his  convergence  up  to  a  certain  limit — for  example,  to  b. 
The  distance  B  b  then  represents  the  negative  part,  and  ab 
the  total  range,  of  relative  convergence. 

§  2.  Desiderata  for  the  Accurate  Measurement  of 
Relative  Convergence. — As  certain  requisites  are  necessary 
for  the  measurement  of  relative  accommodation,  so  certain 
ones  must  also  be  provided  for  the  measurement  of  relative 
convergence.  We  should  have  : 

A.  A  visuometer. 

B.  A  suitable  test  light  in  the  distance  and  a  slight  change 

made  in  the  optometer.  • 

C.  A  table  showing  the  size  of  the  meter  angle  expressed 

in  degrees  with  different  lengths  of  the  base  line. 

D.  A  table  showing  the  angles  of  deflection  caused  by 

prisms. 

E.  Blanks  on  which  the  data  can    be   conveniently    re- 

corded. 

F.  Other  blanks,  the  co-ordinates,  on  which  the  curves 

or  lines  can  be  plotted. 

It  will  be  observed  that  we  are  already  familiar  with  the 
first  four  of  these  desiderata. 

The  blanks  for  recording  relative  convergence  require  a 
word  of  explanation.  These  are  quite  as  necessary  and  also 
as  simple  as  those  for  recording  relative  accommodation. 
The  first  column  on  the  left  shows  the  convergence  in  meter 
angles.  The  second  gives  the  amount  of  accommodation 
actually  exerted,  this  being  dependent  on  the  kind  and  the  de- 
gree of  ametropia,  if  any  exists.  The  third  shows  the  strength 
of  the  adductive  prisms  which  are  overcome.  The  fourth  gives 
the  positive  part  of  the  relative  convergence  when  ex- 
pressed in  meter  angles ;  the  fifth,  the  strength  of  abductive 
prisms ;  the  sixth,  the  negative  part  of  the  relative  con- 
vergence ;  the  seventh  gives  the  total  range,  or  the  sum 
of  the  positive  and  negative  portion  of  the  relative  con- 
vergence. One  of  these  blanks,  filled  out,  is  seen  in  the  next 
section. 


Measurement  of  Relative  Convergence       343 

§  3.     How  to    Measure   Relative   Convergence. — The 

procedure  for  measuring  relative  convergence  is  similar  to 
that  for  relative  accommodation  and  in  theory  is  very  sim- 
ple. That  is,  with  a  given  accommodation  the  strongest 
adductive  prism  shows  the  relative  near  point  of  fusion,  while 
the  strongest  abductive  prism  shows  the  relative  far  point  of 
fusion. 

An  example  will  show  this.  Suppose  we  are  measuring 
the  relative  convergence  of  an  emmetrope  whose  base  line 
is  58  millimeters.  Also  suppose  that  while  viewing  the  test- 
light  6  meters  distant  he  can  overcome  an  adductive  prism 
of  7  degrees  before  each  eye,  or,  what  is  the  same  thing,  10 
degrees  before  one  eye  and  4  before  the  other.  On  consult- 
ing the  table  (page  289),  we  find  a  prism  of  7  degrees 'causes 
a  deflection  of  3.65°. 

But  from  the  table  of  convergence  in  meter  angles  (page 
294)  it  appears  that  when  an  individual  having  a  base  line  of 
58  millimeters  converges  one  meter  angle,  that,  in  degrees, 

is  i°  40'  =  1.66°.     Therefore        „    =2.10  is  the  amount  of 

1.66 

this  positive  relative  convergence  expressed  in  meter  angles 
— that  is,  A  E  n  or  A'  E'  n'  (Fig.  220).  Or  again,  suppose 
that  our  emmetrope,  while  viewing  the  test-light  six  meters 
distant,  can  overcome  an  abductive  prism  of  6  degrees  be- 
fore each  eye,  or  its  equivalent.  Now  from  the  same  table 
of  deflections  we  find  prism  6°  =  3.01.°  Therefore 

3^1  =  1.8  =  A  E  x  or  A7  E'  x'. 
1.66 

The  same  plan  of  course  is  to  be  followed  with  conver- 
gence at  each  meter  angle.  In  making  these  measurements 
we  use  the  optometer  already  described,  turning  the  small 
box  which  is  on  the  long  bar  so  that  the  side  which  faces 
the  patient  presents  to  him  a  vertical  line  with  a  dot  as  the 
object  for  fixation. 

These  illustrations  are  sufficient  to  show  the  general 
principle  involved  and  that  this  part  is  simple  in  the  extreme. 
The  adduction,  as  measured  in  degrees,  is  entered  in  the 
blank  column  opposite  the  point  which  shows  the  corre- 
sponding amount  of  accommodation.  The  abduction  is  t 


344      Representation  of  Relative  Convergence 


entered  in  the  blank  column,  also  opposite  the  figure  for  the 
corresponding  amount  of  accommodation,  and  the  total  of 
the  adduction  and  abduction  is  entered  in  the  last  column. 
For  practical  purposes  this  is  quite  sufficient. 

The  following  is  one  of  the  blanks  referred  to  in  section  2, 
filled  out  with  the  data  furnished  by  the  examination  of  the 
relative  convergence  of  the  emmetrope  already  mentioned. 


Date. 


Relative  Convergence. 


Case  No. 


Name. 
Residence. 


Age. 


Base  Line. 


6J 

U    tJO 

s< 

ta 

^Q 

Adductive 
Prism 
Overcome. 

Positive 
Convergence. 

Abducuve 
Prism 
Overcome. 

Negative 
Convergence. 

Total, 
in 
Meter  Angles 

O 

o 

7 

2.1 

6 

1.8 

3-9 

I 

I 

7 

2.1 

6 

1.8 

~3^6~ 

2 

2 

6 

1.8 

7 

2.1 

3 

3* 

6 

1.8 

6 

1.8 

4 

4 

7 

2.1 

7 

2.1 

4.2 

5 

5 

8 

2.5 

7 

2.1 

4.6 

6 

6 

7 

2.1 

8 

2.5 

4.6 

7 

7 

6 

1.8 

9 

2.8 

4.6 

8 

8 

5 

1-5 

10 

3-i 

4.6 

§4.  Diagrammatic  Representation  of  Relative  Con- 
vergence.— The  earlier  and  more  exact  method  is  to 
represent  the  relative  convergence  on  the  same  system 
of  co-ordinates  and  in  the  same  manner  as  we  plot  the  rela- 
tive accommodation.  For  the  latter  we  have  already  seen 
that  each  one  of  the  squares  from  below  upward  represents 
one  diopter  of  accommodation,  while  each  square  from  the 
left  to  the  right  represents  one  meter  angle  of  convergence. 

Evidently  relative  convergence  can  be  counted  in  exactly 
a  similar  manner,  only  instead  of  reckoning  vertically  from  a 
certain  point  of  the  diagonal  we  count  horizontally  from 
that  same  point  of  the  diagonal.  So  many  squares  to  the 
right  show  the  positive  part  of  the  relative  convergence,  or  so 
many  squares  to  the  left  show  the  negative  part.  (B  780,  p. 


Representation  of  Relative  Convergence      345 

98.)  The  reason  for  this  is,  that  normal  convergence  with 
maximum  accommodation  (positive  relative  accommodation) 
is  of  the  same  character  as  normal  accommodation  with  mini- 
mum convergence  (negative  relative  convergence),  and  the 
reverse.  We  might  therefore  expect  that  with  each  meter 
angle  of  convergence,  if  we  count  from  the  diagonal  to  the 
right,  on  the  abscissa,  we  would  find  that  the  positive 
amount  of  the  relative  convergence  would  coincide  with  the 
relative  accommodation.  But  in  reality  that  is  not  always 
the  case,  as  Nagel  himself  has  shown.  The  fact  is  that  if  we 


If 
13 
12 
11 
1O 
9 
8 
7 
6 
5 
4 
3 

f*m 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

^ 

/ 

/ 

/ 

/ 

/ 

..••' 

/ 

/ 

4r 
/} 

/ 

7 

j 

O    1    2    3  4    5   6   7   8   9   10  II    12  13  14 

FIG.  221. — Lines  showing  the  relative  convergence  as   plotted  in 
a  given  case. 

attempt  to  plot  the  relative  convergence  in  a  given  case, — 
for  example,  in  that  of  the  emmetrope  referred  to, — we  find 
that  the  lines  representing  positive  and  negative  convergence 
are  often  almost  parallel  to  the  diagonal  in  our  system  of 
co-ordinates.  Thus  to  plot  the  relative  convergence  with 
parallel  axes,  we  would  represent  the  positive  part  on  the 
right  of  the  first  horizontal  line — that  is,  about  2.1  squares 
from  the  zero  point,  whereas  the  negative  portion  would  be 
represented  on  a  continuation  of  that  line  to  the  left,  a  dis- 
tance of  1.8  squares.  With  one  diopter  of  accommodation, 


346  Clinical  Measurement  of  Relative  Convergence 

the  positive  part  of  the  relative  convergence  would  be  at  a 
distance  of  2.1  squares  from  the  right  of  the  diagonal,  and 
the  negative  portion  at  a  distance  again  of  1.8  squares  to  the 
left  of  that  diagonal,  when  that  second  abscissa  is  extended 
that  distance  to  the  left.  The  other  amounts  of  positive  and 
negative  relative  convergence  are  indicated  in  the  same  way. 
Figure  221  shows  how  the  figures  given  in  the  above  table 
are  plotted  on  the  system  of  co-ordinates. 

Relative  convergence  may  also  be  represented  by  means 
of  a  diagram.  This  is  seen  in  Fig.  219.  The  amount  of 
normal  convergence  is  shown  by  a  heavy  line  passing  from 
each  eye  to  the  point  for  which  each  is  also  accommodated. 
A  pair  of  lighter  lines  converging  toward  a  nearer  point 
indicates  the  positive  part  of  the  relative  convergence; 
another  pair  of  lines  converging  toward  a  more  distant 
point,  the  negative  part ;  and  the  space  between  these  two 
lines  then  shows,  of  course,  the  total  range  of  the  relative 
convergence.  When  this  space  is  shaded  it  makes  this 
range  rather  more  apparent. 

This  graphic  representation  is  very  convenient  in  showing 
at  a  glance  what  is  intended.  It  has  the  disadvantage  that 
it  cannot  be  carried  beyond  more  than  three  or  four  meter 
angles  without  causing  confusion.  It  is  therefore  em- 
ployed to  give  a  general  view  of  relative  convergence 
only  when  that  is  for  a  few  meter  angles,  as  in  the  figure 
mentioned. 

§  5.  How  is  Relative  Convergence  Measured  for  Clini- 
cal Purposes  ? — The  foregoing  shows  us  how  convergence 
is  measured  and  recorded  with  different  degrees  of  ac- 
commodation, but  here  also  the  objection  arises  that  any 
such  detail  is  not  possible  in  the  routine  of  office  practice, 
and  although  we  have  not  yet  reached  the  clinical  aspect  of 
our  subject,  it  is  well  to  ask  here,  as  we  did  in  regard  to 
relative  accommodation,  how  we  can  obtain  quickly  and 
easily  some  idea  of  relative  convergence  in  a  pair  of  emme- 
tropic  or  ametropic  eyes.  The  process  is  simple.  For  these 
rough  tests  we  do  not  need  a  visuometer  or  optometer  or 
special  blanks  of  any  kind.  We  require  only  the  usual  test- 


Importance  of  Relative  Convergence         347 

light  or  object  in  the  distance,  and  the  Graefe  dot  and  line 
drawn  on  the  test-type  card.  It  is  part  of  the  routine 
examination  by  nearly  every  practitioner  to  ascertain  the 
amount  of  adduction  and  of  abduction  for  the  far  point. 
This  is  nothing  more  than  the  range  of  relative  convergence 
with  relaxed  accommodation.  In  routine  cases  it  is  of  course 
only  necessary  to  ascertain  the  relative  convergence  with 
accommodation  thus  relaxed,  and  then  we  proceed  at  once 
to  the  relative  convergence  and  divergence  with  accom- 
modation at  one-third  of  a  meter.  If  these  rough  tests 
at  the  far  point  and  at  one-third  of  a  meter  show  that 
there  is  any  peculiarity  in  the  range  of  relative  convergence, 
a  similar  test  is  made  when  the  dot  and  line  are  held  at  a 
distance  of  one-fourth  or  one-fifth  of  a  meter,  providing  the 
opportunity  is  then  afforded  for  so  much  detail.  In  most 
cases  it  would  be  an  evident  waste  of  time  to  map  out  its  entire 
range,  but  on  the  other  hand,  in  those  exceptional  instances, 
especially  of  esophoria  or  exophoria,  where  there  is  an 
evident  fault  in  the  power  of  convergence,  accurate  measure- 
ment is  not  only  desirable  but  a  necessity,  if  the  clinician 
expects  to  base  his  diagnosis  upon  reliable  data. 

§  6.  The  Clinical  Importance  of  Relative  Convergence 
is  undoubtedly  as  great  as  that  of  relative  accommodation. 
So  much  has  been  said  already  concerning  the  latter  that  no 
elaboration  of  this  point  is  necessary.  Suffice  it  to  mention 
that  the  clinical  relations  of  convergence  in  this  and  in  other 
forms  will  be  met  with  at  every  turn  and  no  phase  of  our 
subject  is  worthy  of  more  careful  study. 


DIVISION  V. 

Relation  of  both  Accommodation  and  Convergence  to  Torsion 
or  True  Torsion  with  Convergence. 

§  i.  Definition. — Torsion  with  convergence  is  the  tipping 
outward  of  the  upper  ends  of  the  vertical  axes  (  true  torsion  ) 
which  accompanies  convergence.  This  is  in  proportion  to 
the  amount  of  convergence  and  varies  according  to  the  incli- 
nation of  the  visual  plane.  It  is  ordinarily  only  of  a  slight 
degree. 

§  2.  Appliances  for  Measuring  Torsion  with  Conver- 
gence.— When  studying  the  first  group  of  associated  move- 
ments we  were  led  to  consider  the  torsion  made  when  the 
visual  axes  are  in  the  primary  position,  and  we  became  ac- 
quainted with  the  appliances  used  by  Hering,  Donders,  and 
others  for  studying  this  group  of  movements.  It  was  men- 
tioned then  that  the  same  appliances  were  also  used  by  these 
earlier  students  to  determine  the  torsion  which  occurs  in  the 
act  of  convergence.  Before  considering  their  use  in  this  way, 
it  will  tend  to  clearness  and  save  repetition  to  make  one  or 
two  preliminary  observations  which  apply  to  them  all. 

(a}  Classification  of  the  appliances.  At  the  outset,  it  is 
desirable,  if  possible,  to  clear  up  some  of  the  confusion  caused 
by  the  fact  that  different  students  have  given  different  names, 
like  the  "  isoscope,"  "  discs,"  "  clinoscope,"  etc.,  to  different 
instruments  all  of  which  are  intended  to  measure  the  tipping 
in  or  out  of  the  vertical  axes.  Stress  has  already  been  laid 
on  the  fact  that  the  'phorias  are  all  of  a  passive  nature,  and  as 
the  clinoscopes  measure  only  degrees  of  cyclophoria  it  is 
well  to  distinguish  these  instruments  as  a  class  from  the  con- 
verging clinoscopes,  or  the  tortometers,  which  measure  the 
cycloduction  accompanying  convergence.  It  is  true,  most  of 
the  tortometers  are  also  clinoscopes,  but  all  clinoscopes  are 

not  tortometers. 

348 


Appliances  for  Measuring  Torsion  349 

Determination  of  the  plane  in  which  the  visual  axes 
lie.  In  the  definition  of  torsion  with  convergence  it  was 
stated  that  the  degree  of  this  varies  according  to  the  inclina- 
tion of  the  visual  plane.  Evidently  it  is  desirable  to  under- 
stand how  this  visual  plane  can  be  determined,  at  least 
roughly.  Probably  the  simplest  way  and  the  one  which  is 
suitable  for  measurements  made  with  all  of  the  earlier  instru- 
ments is  the  plan  described  by  Le  Conte.  A  horizontal 
line  is  drawn  on  the  wall  opposite  the  observer  and  at  the 
same  height  from  the  floor  as  his  eyes.  Then  the  observer 
closes  the  left  eye,  and  with  the  right  follows  the  line  on  the 
wall  until  the  vision  is  obstructed  by  the  root  of  the  nose, 
tipping  the  head  up  or  down  until  the  line  seems  to  touch  the 
nose  at  its  point  of  greatest  recession.  The  same  experiment 
is  repeated  with  the  other  eye.  This  line  on  the  wall  then 
practically  corresponds  with  the  horizontal  plane. 

Next  we  wish  to  determine  the  visual  planes  which  pass 
through  the  nodal  point  of  both  eyes  and  which  are  inclined 
to  the  horizontal  plane  below  or  above  it  at  certain  definite 
angles.  This  can  be  done  in  two  ways.  One  is  by  arranging 
the  Helmholtz  bit  so  that  the  head  can  be  tipped  backward 
at  certain  angles.  That  gives  the  planes  which  are  inclined 
below  the  horizontal  at  certain  angles.  Or,  by  tipping  the  head 
forward  we  obtain  the  angles  inclined  above  the  horizontal. 
It  is  possible,  however,  to  determine  the  planes  of  the  visual 
axes  in  another  way.  On  the  wall  which  is  in  front  of  the 
observer,  and  at  a  distance  of  at  least  six  meters,  we  lay 
out  vertically  a  tangent  scale,  the  radius  of  that  arc  being  the 
distance  from  the  wall  to  the  observer,  and  the  nearest  point 
on  the  scale  being  that  at  which  the  horizontal  visual  plane 
intersects  this  vertical  tangent  scale  at  right  angles.  At 
points  of  say  ten  degrees  apart,  above  and  below  the  horizon- 
tal plane,  horizontal  lines  are  drawn  across  the  wall.  By  fol- 
lowing these  lines  along,  first  with  one  eye,  and  then  with 
the  other,  until  the  view  is  obstructed  by  the  nose  or  by  the 
brow,  we  determine  the  position  of  a  given  visual  plane  in 
the  manner  described  for  determining  the  position  of  the 
horizontal  plane. 

It  is  worth  while  to  glance  thus  at  the  methods  which  have 


350  Appliances  for  Measuring  Torsion 

been  used  for  determining  these  different  planes  in  order  that 
we  may  obtain  more  definite  data  concerning  torsion  with 
convergence  than  those  which  have  been  found  by  earlier  in- 
vestigators, and  upon  which  we  now  rely.  It  is  evident, 
however,  that  any  such  methods  are  adapted  to  the  laboratory 
only.  It  is  for  this  reason  especially,  that  it  seemed  desir- 
able to  adapt  the  tortometer  to  the  ordinary  perimeter,  in 
order  that  when  the  arc  is  vertical  the  instrument  can  be  placed 
at  once  in  any  visual  plane  desired.  This  has  been  done  and 
the  appliance  will  be  described  in  a  subsequent  section. 

(c]  Only  young  subjects  or   those  with  comparatively 
good  power  of  convergence  and  accommodation  are  suitable 
for   the  measurement    of    torsion   with  the  higher   degrees 
of    convergence.      This    statement    is   self-evident.      It   is 
true  that  we  can  measure  the  torsion  which  occurs   with 
accommodation  up  to  the  point  of  clear  vision  for  that  indi- 
vidual as  well  as  for  one  who  has  a  large  range  of  this  kind.     It 
is  also  true  that  by  placing  convex  glasses  before  the  eyes  the 
difficulties  in  the  problem  can  be  to  a  great  extent  eliminated, 
but  as  soon  as  we  thus  change  artificially  the  degree  of  ac- 
commodation   which  naturally  goes   with  a    corresponding 
amount  of  convergence,  we  bring  still  another  factor  into  the 
problem  and  make  the  results  more  uncertain.     Therefore  in 
seeking  for  a  basis  of  physiological  experiment  we  require 
always  young  subjects  whose  power  of  accommodation  and 
convergence  is  still  large. 

(d)  In  all  these  measurements  of  torsion   it  should  be 
understood    that  the  methods  are  more  difficult  than    the 
measurements  of  relative  accommodation  or  relative  conver- 
gence, and  the  results  are  therefore  more  liable  to  error.    Even 
when  we  have  the  best  of  laboratory  appliances  and  also  sub- 
jects well  trained  to  physiological  experiment,  it  must  be  con- 
fessed that  the  measurement  of  torsion  is  by  no  means  an  easy 
matter.     Evidently,  therefore,  measurements  which  are  made 
in  the-eonsultingroom  and  upon  the  average  patient  are  often 
far  from  satisfactory.     It  is  a  field  in  which  improvement  in 
method  is  still  much  to  be  desired.      On  the  other  hand, 
such  facts  as  we  have,  indicate  that  this  factor  in  binocular 
vision  is  quite  as  important  as  is  accommodation  or  con- 


Appliances  for  Measuring  Torsion  351 

vergence.  While  this  frank  statement  of  the  difficulty 
of  its  measurement  is  necessary,  that  should  not  deter 
us  from  efforts  to  be  more  exact  in  this  part  of  our 
examinations. 

The  different  appliances  with  which  this  torsion  can  be 
measured  are : 

(A)  The  arrangement  suggested  by  Hering  (Fig.  182).    In 
studying  torsion  which  occurs  with  parallel  visual  axes  the 
vertical  lines  are  of  course  placed  at  a  distance  of  six  meters 
or  more,  but  when  convergence  is  brought  into  action,  its 
degree  is   measured  either  by  approaching  the  blackboard 
or  by   having   a   point   placed    between   the   observer  and 
the  vertical  lines  so  arranged  that  the  distance  from  this 
point  to  the  observer  can   be   easily  measured.     Hering's 
method    is   of   special  interest  for  the   reason   that  it  was 
followed  by  Landolt  in  the  studies  which  he  made  of  torsion 
with  convergence.     His  results  will  be  given  later. 

(B)  The  appliance  of  Bonders  which  he  called  the  iso- 
scope  was  used  for  the  purpose,  as  we  have  already  seen,  of 
determining  the  amount  of  tipping  which  the  vertical  axes 
make  while  the   visual  axes  are  in   the   primary  position. 
But  what  interests  us  here  is  that  the  torsion  which  occurs 
with  convergence  was  described    by  Bonders  at  the  same 
time  that  he  described  and  figured  the  isoscope.   His  method, 
although  slightly  different    from  those  of  Hering   and  of 
Volkmann,    shows   in    general  that   the  differences  in   the 
results  are  only  slight. 

(C)  Volkmann's    discs.      While    studying    the   position 
which   the   vertical   axes  tend  to  assume  when  the  visual 
axes  are  in  the  primary  position  (Chapter  II,  Section  5),  we 
found  that  Volkmann's  discs  could  be  used  to  advantage 
in  making  these  measurements.      In  Stevens'  later  model  of 
his  clinoscope  (Fig.  178)  he  shortened  the  tubes  to  permit 
more  convergence.    But  as  sufficient  convergence  of  the  tubes 
could  not  be  obtained  in  that  way,  it  seemed  desirable  to 
mount  the  same  discs  in  quite  another  manner  (Fig.   179). 
As  we  were  then  considering  only   the  torsion  which  can 
occur  with  the  visual  axes  in  the  primary  position,  no  men- 
tion was  made  of  the  method  of  measuring  the  torsion  which 


352  Appliances  for  Measuring  Torsion 

accompanies  convergence.  It  is  therefore  proper  to  explain 
that  at  this  point. 

It  can  be  shown  best  by  an  example.  Suppose  it  is 
desired  to  measure  the  torsion  which  takes  place  in  the  eyes 
of  a  given  individual  when  he  converges  and  accommodates 
to  a  point  in  the  horizontal  plane  which  lies  one-third  of  a 
meter  from  the  center  of  his  interocular  base  line.  Having 
noticed  that  the  long  arm  of  the  instrument  is  practically 
level,  as  shown  by  the  marks  on  the  two  supports,  the  fixa- 
tion point  P  is  slid  along  this  arm  to  the  point  which  is 
marked  as  being  one-third  of  a  meter  from  the  base  line. 
The  subject  of  the  test  then  adjusts  his  head  in  the  rest,  in 
such  a  manner  that  his  eyes  are  on  a  level  with  the  fixation 
point.  He  then  closes  one  eye — the  left,  for  example — and 
sighting  over  the  fixation  point,  the  left-hand  disc  with  its 
vertical  diameter  is  slid  along  the  arm  on  which  it  rests  until 
the  center  of  that  disc  is  in  line  with  the  fixation  point  P. 
In  the  same  way  the  right  eye  is  closed  and  the  right- 
hand  disc  is  brought  in  line  with  the  fixation  point  P 
and  the  macula  of  the  left  eye.  Then  both  eyes  are  opened, 
and  when  the  fixation  point  is  looked  at,  if  the  diameter  on 
each  disc  has  been  set  in  the  proper  position,  the  observer 
sees  three  images  of  the  Volkmann  disc,  the  middle  one  of 
course  being  formed  by  the  fusion  of  the  other  two.  The 
number  of  degrees  which  it  is  necessary  to  tip  the  upper  end 
of  the  axis  of  each  of  these  discs,  in  order  to  have  them  fuse 
into  a  single  vertical  diameter,  is  the  measure  of  the  degree 
of  torsion  which  each  eye  undergoes.  If  it  is  desired  to 
measure  torsion  with  this  instrument  in  planes  inclined 
slightly  above  or  below  the  horizontal,  that  can  be  done  by 
varying  the  height  of  one  support  or  the  other,  as  shown  by 
the  dotted  lines  in  the  illustration  (Fig.  179). 

(D)  Le  Conte's  Plan  J  was  to  prepare  a  plane  about  75 
centimeters  long  by  half  as  wide,  and  divide  it  into  two 
squares,  one  black  and  the  other  white,  Fig.  222.  Each  half 
was  then  subdivided  into  smaller  squares  by  vertical  and 

1  Although  these  squares  were  arranged  by  Helmholtz  (B.  275,  Plate  4),  the 
details  of  their  use  were  elaborated  by  Le  Conte. 


Le  Conte's  Method 


353 


horizontal  lines.  Finally,  a  small  circle  was  drawn  near  the 
center  of  each  large  square.  The  method  of  showing 
the  existence  of  torsion  with  these  squares  is  as  follows:  the 
plane,  exactly  vertical,  is  placed  on  a  table  at  a  distance  of 
60  or  80  centimeters  from  the  observer.  The  latter  rests 
his  chin  on  the  table  immediately  in  front  of  these  two 
squares,  arid  at  such  a  height  that  the  level  of  the  eyes  is 
exactly  on  the  level  of  the  small  circles.  The  eyes  are 
then  forcibly  converged  until  the  right  eye  is  looking  at 
the  left  circle  and  the  left  eye  at  the  right  circle.  This  is 
accomplished  with  difficulty  at  first,  but  becomes  easier 


FIG.     222. — Arrangement  of  horizontal  and  vertical   lines    for   testing   the 
amount  of  torsion  with  convergence. 

with  some  practice.  Under  such  circumstances  the  ob- 
server will  see  that  the  lines  which  are  actually  vertical 
seem  to  diverge  upward  forming  a  V.  Moreover,  the  angle 
at  which  the  vertical  lines  stand  increases  more  and  more 
in  proportion  to  the  degree  of  convergence — that  is,  the 
vertical  lines  which  are  actually  before  the  right  eye  are 
then  apparently  transposed  to  the  left  side.  Besides, 
their  upper  ends  are  tipped  to  the  left,  showing  that  the 
upper  end  of  the  vertical  axis  of  the  right  eye  is  tipped 
a  corresponding  number  of  degrees  to  the  right.  By  chang- 
ing the  distance  between  these  lines  and  the  eyes,  it  will  be 
observed  that  the  former  seem  to  diverge  more  and  more 
the  closer  they  are  approached. 
23 


354  The  Tortometer 

This  method  of  estimating  the  amount  of  torsion  with 
convergence  was  simple  but  tedious.  It  was  necessary  to 
draw  one  set  of  squares  after  another,  the  lines  which 
composed  one  square  being  inclined  to  those  of  the  other 
squares  at  a  known  angle.  When  these  lines  appeared  to 
the  converging  eyes  to  be  vertical, — that  is,  when  the  angle 
at  which  the  lines  actually  converged  upward  was  balanced 
by  the  angle  which  the  vertical  axes  of  the  eyes  converged 
downward,  the  degree  of  torsion  was  known.  An  example  of 
one  of  these  trial  figures  given  by  Le  Conte  is  shown  in 
Fig.  223. 


FIG.  223. — Lines  drawn  at  an  angle  in  order  to  have  them  appear  vertical 
when  the  eyes  are  in  extreme  convergence. 

(E)  Author's  Arrangement  of  Le  Conte's  Squares 
(the  Tortometer). — While  Le  Conte's  squares  constituted 
the  best  device  up  to  his  time,  for  determining  torsion  with, 
a  given  degree  of  convergence,  he  left  much  to  be  done  in 
simplifying  the  method.  After  many  vain  attempts  to 
obtain  with  these  squares  data  which  were  fairly  constant, 
a  number  of  improvements  in  his  plan  gradually  suggested 
themselves.  They  took  the  form  of  a  device  for  measuring 
the  degree  of  torsion  which  might  be  called  a  tortometer. 
As  this  is  nothing  more  than  a  mechanical  adaptation  of 
Le  Conte's  squares,  the  amount  which  the  upper  ends  of 
the  vertical  lines  of  the  squares  converge  corresponds  to  the 
amount  which  the  upper  ends  of  the  vertical  axes  of  the 


The  Tortometer 


355 


eyes  diverge.  In  the  arrangement  referred  to,  the  object 
looked  at  is  not  exactly  the  two  sets  of  squares,  but  instead, 
it  has  been  found  more  convenient  to  use,  before  one  eye, 
vertical  black  lines  on  a  white  ground,  and  before  the  other, 
vertical  white  lines  on  a  black  ground.  All  the  horizontal 
lines  except  one  are  omitted,  for  even  a  trained  observer 
may  be  confused  by  their  presence.  By  a  simple  mechanism 


FIG.   224. — Tortometer  of  the   author  arranged  for  measuring  torsion 
in  the  horizontal  plane. 

(Figs.  224  and  225)  these  vertical  lines  may  be  tipped  into 
any  position  desired  and  the  angle  between  them  can  then 
be  read  off  on  the  short  arc  (D).  In  order  to  accomplish 
this,  each  of  the  two  cards  is  fixed  in  a  frame  of  brass  about 
eight  or  ten  centimeters  square.  The  upper  edge  of  each 
of  these  frames  is  attached  to  a  band  (C)  of  flexible  steel, 
one  centimeter  wide  and  two  or  three  millimeters  thick. 
This  band,  in  turn,  is  connected  with  a  horizontal  rod 
which  has  a  nut  (B)  on  the  right  side.  By  turning  this  nut, 


356  The  Tortometer 

the  angle  between  the  lines  can  be  increased  or  diminished. 
The  point  F,  at  which  the  visual  axes  are  converged,  is  a 
ball  which,  being  attached  to  a  support,  can  be  slid  back- 
wards and  forwards  any  distance  desired. 

This  whole  arrangement  may  be  attached  by  a  slot  on  its 
posterior  surface  to  any  support.  One  such  support  is 
the  horizontal  bar  already  described  as  a  part  of  the  op- 
tometer  for  measuring  relative  accommodation  and  conver- 
gence (Fig.  224).  When  mounted  in  this  way,  we  measure 
with  it  the  torsion  with  convergence  in  the  horizontal 


FIG.  225. — Tortometer    of    the   author  ar- 
ranged for  measuring  torsion  in  various  planes. 

plane.  Or  the  two  cards  thus  mounted  can  also  be  attached 
to  one  of  the  carriers  on  the  concave  surface  of  the  ordinary 
perimeter,  and  we  can  then  measure  the  torsion  with  con- 
vergence in  planes  inclined  either  below  or  above  the 
horizontal  (Fig  225). 

The  method  of  measuring  torsion  with  this  arrangement 
is  simple.  First,  the  head  is  brought  to  the  proper  height 
and  steadied  by  resting  the  teeth  on  the  wooden  bit,  or  a 
head-rest  with  the  forehead  piece  can  be  used. 

Second,  the  horizontal  plane  of  the  eyes  is  determined  in 
the  manner  described  on  page  349  either  with  the  assistance 


The  Tortometer  357 

of  a  line  drawn  on  the  wall  opposite  the  observer,  or  by  some 
substitute  for  it. 

Third,  the  person  is  then  directed  to  look  at  the  fixation 
point  (  F)  (whose  distance  from  the  eyes  can  be  changed 
at  will)  until  a  part  or  all  of  the  vertical  lines  on  the  right 
side  appear  to  be  on  the  left,  and  the  reverse. 

Fourth.  When  this  is  accomplished,  the  lines  when 
actually  vertical  seem  to  converge  downward  if  the  eyes  are 
normal.  Then,  by  turning  the  nut  (B)  the  cards  are  tipped 
gradually,  so  that  the  lines  actually  converge  upward. 
When  the  point  is  reached  where  they  seem  to  the  observer 
to  be  vertical,  the  examiner  reads  off  on  the  arc  (D)  the 
angle  which  the  two  sets  of  lines  make  with  each  other. 
Half  of  that  arc  is  the  amount  which  the  upper  end  of  each 
eye  tips  outward,  when  converged  to  the  distance  which  the 
fixation  point  F  is  from  the  center  of  the  base  line. 

The  amount  of  torsion  present  when  the  eyes  are  elevated 
or  depressed  can  be  measured  as  easily  with  the  tortometer 
here  described  as  can  torsion  in  the  horizontal  plane.  As  a 
preliminary  step,  it  is  necessary  to  bring  the  center  of  the 
tortometer  (that  is,  the  point  which  is  in  the  middle  of  the 
adjoining  edges  of  the  black  and  white  cards)  to  correspond 
with  the  center  of  the  arc  of  the  perimeter  (  Fig.  225  ).  The 
four  succeeding  steps  are  the  same  as  those  described  for 
measuring  torsion  in  the  horizontal  plane.  After  that  has 
been  determined  for  a  certain  degree  of  convergence,  as 
regulated  by  the  position  of  the  point  for  fixation  (F),  the 
cards  are  slid  upward  on  the  arc  a  certain  distance,  say 
ten  degrees,  and  the  reading  taken  ;  afterwards  at  twenty,  at 
thirty  degrees,  etc. ;  or  the  cards  are  slid  downward  and 
similar  readings  taken  at  successive  points. 

In  regard  to  the  actual  usefulness  of  this  arrangement  it  must 
be  said  that  in  spite  of  all  the  care  which  can  be  exercised  in 
its  construction,  and  its  use  even  by  an  intelligent  patient,  the 
results  obtained  are  not  always  constant.  This  may  be  due 
to  the  fact  that  different  persons,  even  when  about  the  same 
age,  probably  make  slightly  different  degrees  of  torsion.  It 
must  also  be  said  that  even  the  same  person  will  show  at  dif- 
ferent times  a  perceptible  difference  in  the  amount  of  torsion, 


358 


Scale  on  the  Tortometer 


no  matter  in  what  way  it  is  measured.  These  sources  of 
error  should  be  mentioned.  But  on  the  other  hand,  they 
are  not  greater  than  those  which  occur  with  the  measure- 
ments of  accommodation*  or  of  convergence,  nor  those  which 
we  find  in  any  similar  physiological  experiments. 

As  the  lines  on  these  cards  are  inclined  more  and  more,  the  question  arises 
of  how  we  can  measure  the  angle  which  they  (and  the  cards)  make  with  each 
other.  It  should  therefore  be  referred  to. 


FIG.  226. — Diagram  showing  how  to  calculate  the  angle  at  which  the 
lines  converge  in  the  tortometer. 

Let  ABCD,  Fig.  226,  be  the  card  carrier  in  its  primary  position,  and  AB'C'D' 
be  its  position  after  rotating  through  an  angle  P.  To  calibrate  an  instrument 
of  this  kind,  the  simplest  way  is  to  insert  a  card  back  of  the  card  carrier 
ABCD,  then,  taking  a  sharp  pencil  and  using  the  bottom  (CD)  of  the 
card  as  a  ruler,  draw  a  line  ECDF.  It  is  obvious  that  when  the  card  carrier 
has  moved  to  a  new  position,  making  any  angle,  as  P,  with  the  old  one,  that 
the  side  B'D'  will  cut  the  line  EF  at  some  point,  as  G.  Then,  setting  the  card 
carrier  at  different  angles,  mark  off  different  points  on  EF  for  the  various  angles. 

A  more  accurate  way  is  to  calculate  first  what  the  distances  on  CD  are  for 
different  angles.  To  do  this,  from  B'  drop  a  line  perpendicular  to  AB  and 
extend  it  to  CD,  then  since  CD  is  parallel  to  AB,  -HI  is  perpendicular  to  CD. 

The  angle  GB'H  is  equal  to  the  angle  C'AC,  which  equals  P,  since  the 
sides  of  the  angles  are  mutually  parallel  and  run  in  the  same  direction. 

Let  any  distance,  as  GD,  be  denoted  by  K.  Let  the  side  BD  of  the  right 
angle  be  denoted  by  L,  and  let  the  distance  AB,  from  the  center  of  rotation  to 
the  farther  end  of  the  card  carrier,  be  denoted  by  M. 

Then 

K  =  GH  +  HD  (i) 

CH  =  (HB')  (tanP)  (2) 

HB'  =  HI  -  IB'  =  L  -  IB'  (3) 


Torsion  in  the  Horizontal  Plane  359 

IB'   =  ( AB'i  ( sin    IAB')   =  ( M )  (sin  IAB'),  but  the  angle  IAB'  =  the 
angle  C'  AC,  since  their  sides  are  mutually  perpendicular. 
Hence 

IB'  =  M  sin  P.     Put  this  value  in  (3). 
HB'=  L  —  sin  P.     Put  this  value  in  (2). 
GH  =  (  L  —  M  sin  P )  tan  P.     Put  this  value  in  (i). 
K  =  ( L  -  M  sin  P  )  tan  P  +  HD  (4) 

To  find  HD,  observe  that 

HD  =  IB  =  AB  -  AI  =  M  -  AI  =  M  -  AB'  cos  P  =  M  -  M  cos  P. 
Put  this  value  in  (4). 

It  is  unnecessary  to  follow  the  intermediate  steps  of  this  calculation,  and  it 
must  suffice  here  to  say  that  we  arrive  ultimately  at  the  expression: 
jr    __     M  cos  P  —  M  sin  P 
—  sin  P 

Having  thus  glanced  at  the  structure  and  working  of 
several  appliances  used  for  measuring  torsion  with  con- 
vergence we  naturally  ask  which  among  them  all  is  the 
best.  Unfortunately  no  one  instrument  or  method  can  be 
selected  as  being  superior  in  every  way.  For  persons  of  in- 
telligence the  method  of  Le  Conte  is  not  only  the  most 
reliable  but  the  most  expeditious.  For  that  reason  the 
attempt  was  made  to  simplify  it,  and  adapt  it  not  only  to 
laboratory  but  to  clinical  use. 

§  4.  How  Great  Is  the  Torsion  with  Convergence  in 
the  Horizontal  Plane? — It  will  be  easier  to  understand 
the  results  of  these  measurements,  no  matter  in  what  way 
they  are  made,  if  it  is  repeated  that  the  amount  of  torsion 
with  convergence  depends  upon  two  distinct  factors.  One 


A  B  C  C  B'  A' 


FlG.  227. — Schematic  view  of  the  position  of  the  vertical  axes  with  increas- 
ing convergence  in  the  horizontal  plane.  The  actual  vertical  is  drawn  with  a 
dotted  line.  A  A'  position  with  parallel  visual  axes;  B  B'  with  moderate 
convergence  ;  C  C'  with  extreme  convergence. 

is  the  amount  of  convergence  and  of  accommodation  which 
is  exerted  by  the  individual,  and  the  other  factor  is  the 
position  of  the  plane  in  which  the  visual  axes  lie — that  is, 
whether  they  are  in  the  horizontal  plane  or  in  a  plane 


360  Torsion  in  the  Horizontal  Plane 

inclined  below  or  above  the  horizontal.  In  considering  the 
results  of  these  measurements  we  naturally  ask  first : 

What  amount  of  torsion  does  each  eye  undergo  when 
the  visual  axes  lie  in  the  horizontal  plane  and  cross  in  the 
median  plane? 

This  has  been  found  by  Landolt's  measurements  to  be  as 
follows 

Convergence  Torsion  in  the 

in  Horizontal 

Degrees.  Plane. 

5  I    30' 

6  I  45' 

7  2     5' 

8  2     5' 

9  2  10' 

10  2    30' 

11  2    40' 

12         2  55' 

13         3  20' 

14         3  30' 

16         3  55' 

18         4  50' 

20         5  40' 

25         5  52' 

30         6  50' 

The  gradual  tipping  of  the  upper  end  of  the  vertical  axes 
as  the  object  approaches  in  the  horizontal  plane  is  seen  in  an 
exaggerated  form  in  Fig.  227. 

§  5.  How  Great  Is  the  Torsion  with  Convergence 
above  or  below  the  Horizontal  Plane  ? — We  have  just 
seen  that  as  the  convergence  in  the  horizontal  plane  in- 
creases, the  upper  end  of  the  vertical  axis  of  each  eye  tends 
to  tip  outward.  We  come  now  to  another  fact — namely, 
that  the  tipping  outward  of  the  upper  end  of  the  vertical 
axes  usually  increases  as  the  plane  of  the  axes  of  vision  is 
elevated.  It  is  true  that  when  we  examine  any  of  these 
series  of  measurements,  like  the  data  given  by  Landolt 
(B.  820,  p.  662),  for  example,  we  notice  that  the  increase  in 
the  amount  of  tipping  is  not  always  in  proportion  to  the 
amount  of  convergence.  This  is  apparently  contradictory, 
but  any  one  who  is  familiar  with  such  laboratory  measure- 
ments appreciates  that  at  least  some  irregularity  in  the 


Torsion  in  an  Oblique  Plane 


results  is  the  rule  rather  than  the  exception.  No  matter  how 
intelligent  the  subject  of  the  measurement  may  be  or  how 
accurately  the  measurements  are  made,  such  irregularities 
are  apt  to  occur.  It  is  also  true  that  the  experiments  made 
by  different  investigators  give  slightly  different  results,  but 
they  accord  sufficiently  to  show  that  Landolt's  figures  fur- 
nish a  standard  by  which  to  measure  the  amount  of  torsion 
which  is  normal.  Accordingly  this  part  of  his  table  is 
reproduced  below. 


Above  the  Horizontal  Plane. 


Below  the   Horizontal  Plane. 


i.2» 

f  Kf             25              20              I0 

i  §  U 

O  «)  nj 

O  biro 

10        .           20                   25                   30                  ^O 

5 

2 

i     5' 

i 

I 

i 



i 

i 

6 

2   30' 

2 

i     5' 

I     2O' 

i 

— 

i  10' 

i  30' 

7 

3 

2   30' 

2   30' 

I     30' 

i  30' 

i 

i  30' 

i  30' 

8 

3 

2    20' 

2    20' 

I     3O' 

i  30' 

i 

i  30' 

I  35' 

9 

3   1O 

2    20' 

2   30' 

I     4O' 

i  30' 

i  15' 

i  20' 

i  35' 

10 

4  20' 

3    30' 

3 

2 

i  40' 

i  30' 

i  5o' 

i  40' 

ii 

5 

3    30' 

3 

2 

i  50' 

— 

2 

i  5C-' 

12 

5 

3  4o' 

3  10' 

2    IO' 

2 

2 

i  30' 

13 

7 

3  4o' 

3  40' 

2    3O' 

2 

2    IO' 

i  30' 

14 

7  30' 

4 

3  40' 

2  40' 

2       5' 

2   20' 

i  30' 

16 

8 

5  30' 

4     5' 

3 

2    IO' 

2   30' 

2 

18 

9 

5  30' 

5 

4 

3 

2    50' 

2  30' 

20 

it 

7  3°' 

6 

4  30' 

3  30' 

3  10' 

245' 

25 

15 

8 

7 

4  50' 

4 

3  30' 

2 

30 

16  30' 

10 

8 

5  So' 

4 

4 

I 

§  6.  How  to  Plot  Torsion  in  a  Given  Plane. — This 
can  be  done  with  the  ordinary  system  of  coordinates  in  the 
same  manner  as  we  have  already  plotted  relative  accommo- 
dation. A  convenient  method  is  to  have  the  ordinates  repre- 
sent the  increase  in  convergence. (and  accommodation),  and 
the  abscissas  represent  the  increase  in  the  amount  of  torsion 
from  zero  to  eight  or  ten  degrees.  As  it  is  possible  in  lab- 
oratory work  to  estimate  the  amount  of  torsion  to  half  a 
degree,  as  Landolt  has  done,  each  space  between  the  ab- 
scissas can,  for  such  purposes,  be  counted  as  thirty  sec- 
onds. For  clinical  purposes,  however,  such  accuracy  is 
impracticable. 


362  Relative  Torsion 

§  7.  Relative  Torsion  is  the  amount  which  the  upper 
end  of  the  vertical  axes  tips  in  or  out  beyond  the  amount 
which  normally  occurs  with  a  given  degree  of  convergence 
in  a  given  plane. 

Just  as  we  have  found  that  we  can  measure  a  positive 
and  a  negative  portion  in  the  range  of  accommodation  with 
relation  to  convergence,  and  the  reverse,  so  can  we  also  dis- 
cover a  positive  and  negative  portion  of  the  range  of  torsion 
with  relation  to  both  accommodation  and  convergence. 
These  measurements  can  be  made,  approximately  at  least, 
with  the  converging  clinoscope  (Fig.  179),  entire  diameters 
being  used  on  the  discs. 

§  8.     Desiderata  for  Testing  Relative  Torsion  are : 

A.  A  tortometer  (converging  clinoscope). 

B.  A  table  showing  the  angle  of  torsion  with  a  given 
degree  of  convergence  in  a  given  plane  (Landolt). 

C.  A  table  expressing  degrees  in  terms  of  the  meter 
angle. 

D.  A  blank  on  which  to  enter  the  measurements  made. 
The  heading  for  these  blanks  should  have  space  for  the 

number,  name,  etc.  It  should  also  indicate  whether  the  meas- 
urements made  are  in  the  horizontal  plane,  or  if  these  are 
made  in  any  plane  above  or  below  the  horizontal,  that  should 
be  stated  also.  The  first  column  to  the  left  indicates  what  is 
the  degree  of  convergence  as  expressed  in  meter  angles. 
The  second  column  shows  what  the  convergence  is  in 
degrees.  Knowing  the  base  line  it  is  easy  to  ascertain  what 
that  number  of  degrees  is  from  the  table,  page  294.  The 
third  column  shows  what  the  normal  torsion  is  with  that 
degree  of  convergence.  When  this  is  expressed  in  degrees 
we  can  copy  it  directly  from  Landolt's  table. 

In  the  fourth  column  we  enter  the  amount  of  positive 
relative  torsion.  This  is  obtained,  as  has  been  already 
stated,  by  finding  first  what  is  the  total  amount  which  the 
vertical  axis  seems  to  tip  outward.  From  that  amount  we 
deduct  the  amount  of  torsion  which  we  find  from  Landolt's 
table  to  be  the  normal  amount  of  tipping  outward  for  that 
degree  of  convergence,  and  the  difference  between  the  two 
is  the  positive  part  of  the  relative  torsion.  In  a  similar 


Disturbances  of  Torsion  363 

manner  we  obtain  the  negative  part  of  relative  torsion. 
That  is  entered,  in  the  fifth  column.  The  last  column 
shows  the  total  amount  of  relative  torsion.  This,  of  course, 
is  obtained  by  adding  together  the  positive  and  the  nega- 
tive part.  It  is  well  to  leave  a  portion  of  the  blank  on 
which  to  enter  memoranda  relating  to  these  different 
measurements. 

§  9.  Imperfection  of  our  Data. — This  reference  to  rela- 
tive torsion  and  to  desiderata  for  testing  it  has  been  made 
rather  for  the  sake  of  completeness  than  because  of  the  possi- 
bility of  great  exactness  with  the  methods  now  at  our  com- 
mand. The  aim  has  also  been  to  indicate  a  plan  on  which 
further  studies  of  these  subjects  could  be  prosecuted. 

But  it  must  be  admitted  that  our  actual  data  concerning 
normal  torsion  with  convergence  are  unfortunately  few  and 
imperfect.  Still  more  meager  is  our  knowledge  of  relative 
torsion  with  convergence.  But  painstaking  measurements  of 
even  moderately  intelligent  subjects  give  results  which  are 
fairly  constant,  and  it  is  not  too  much  to  hope,  therefore, 
that  with  improved  appliances  this  phase  of  our  subject  may 
become  clearer,  as  its  apparent  importance  is  more  fully 
appreciated. 

§  10.  The  Object  of  Torsion  with  Convergence  is 
not  clearly  understood.  There  is  no  apparent  reason,  for 
example,  why,  when  we  look  at  a  near  point,  accommo- 
dation and  convergence  should  not  be  sufficient  without  this 
additional  wheel-like  motion.  Indeed,  it  is  difficult  for  us 
to  understand  how  that  can  occur  without  producing  some 
diplopia.  Various  studies  of  this  question  have  been  made, 
and  they  indicate,  in  general,  that  the  torsion  which  occurs 
with  convergence  does  assist  in  producing  stereoscopic 
vision.  There  still  remains,  however,  very  much  for  us  to 
learn  concerning  this  subject.  Although  we  do  not  under- 
stand the  reason  for  this  motion,  the  practical  fact  remains 
that  when  it  is  disturbed  either  purposely  or  by  pathological 
conditions,  the  results  then  appear  of  importance. 

§  ii.  What  Happens  if  Torsion  with  Convergence  is 
Artificially  Disturbed  ? — We  have  thus  far  no  means  of 
controlling  torsion,  as  readily  as  cycloplegics  or  myotics 


364  Disturbances  of  Torsion 

control  the  accommodation,  or  as  prisms  affect  the  converg- 
ence. We  have  already  seen,  however,  that  even  with 
parallel  axes  any  disturbance  of  the  rotation  of  the  globe 
on  its  fore-and-aft  axis,  as  when  made  in  the  interest  of 
single  vision,  is  accompanied  by  discomfort.  If  we  make  use 
of  a  clinoscope  so  constructed  as  to  permit  sufficient  con- 
vergence, it  is  possible  to  show  in  a  still  more  marked  de- 
gree how  great  is  the  discomfort  produced  by  even  a  slight 
disturbance  of  torsion.  With  this,  the  experiment  can  be 
made  as  for  parallel  axes,  of  gradually  turning  the  vertical 
lines  too  far  out  of  place,,  The  effort  at  torsion  which 
the  eyes  make  to  fuse  these  lines  usually  shows  itself  in  a 
sense  of  discomfort  which  is  very  noticeable.  The  disturb- 
ing effect  of  the  torsion  is  familiar  to  every  ophthalmolo- 
gist when  certain  glasses  are  prescribed,  especially  if  these 
are  cylinders  of  considerable  strength.  The  patients  often 
complain  that  vertical  lines  diverge  downward  or  upward, 
thus  making  a  page  seem  broader  above  or  below,  or  vari- 
ous similar  distortions  may  occur.  After  a  time  the  eye 
and  perhaps  the  brain,  or  both,  adjust  themselves  to  this 
new  position,  and  the  annoyance  ceases.  In  like  manner, 
in  cases  of  simple  astigmatism  where  accommodation  and 
convergence  are  normal,  proper  corrective  glasses  relieve 
the  discomfort.  If,  now,  the  axis  of  one  or  both  cylin- 
ders be  changed,  even  a  few  degrees,  a  considerable  amount 
of  inconvenience  is  often  experienced,  due,  so  far  as  we 
know,  to  an  unnatural  torsion,  as  the  eye  tries  to  adapt 
itself  to  the  new  axis  of  the  glass.  The  light  circular 
frames  which  are  sold  with  cylinders  and  can  be  lent  to 
patients  are  very  useful  in  making  these  interesting  and 
instructive  experiments. 

The  manner  in  which  torsion  can  be  disturbed  is  also 
seen  by  a  simple  example.  Let  us  suppose  that  we  have 
to  do  with  a  marked  degree  of  astigmatism.  When  meas- 
uring its  degree  and  its  angle,  we  test  the  eyes  with  objects 
placed  in  the  horizontal  plane  and  with  the  visual  axes 
parallel.  Then  we  prescribe  glasses  which,  under  such 
circumstances,  give  the  best  correction.  When,  however, 
that  same  person  exerts  a  very  considerable  amount  of 


Value  of  Such  Measurements  365 

convergence  in  the  same  horizontal  plane,  or  when  he  lifts 
the  eyes  so  that  the  visual  axes  are  in  a  plane  which  is 
inclined  to  the  horizontal  plane  at  a  considerable  angle, 
then,  in  the  natural  torsion  which  accompanies  either  of 
these  movements,  the  angle  at  which  the  glasses  were  set 
no  longer  coincides  with  the  angle  of  the  astigmatism  of  the 
eyes.  There  are  several  occupations  which  require  the 
elevation  of  the  eyes  in  this  way,  as,  for  example,  when  a 
clerk  is  obliged  to  look  up  to  read  labels  on  shelves,  or 
when  an  accountant,  bending  the  head  down  toward  one 
book  which  is  immediately  in  front,  looks  occasionally  at 
another  book  situated  also  on  the  same  table  but  a  little 
distance  away. 

§  12.  What  is  the  Clinical  Value  of  Any  Such  Facts 
or  Measurements  ? — This  question  may  well  be  asked, 
especially  in  view  of  the  small  amount  of  torsion  which  can 
be  detected,  even  at  the  maximum.  From  the  foregoing  it 
would  appear  that  in  practice: 

1st.  We  must  take  torsion  into  account,  together  with 
accommodation  and  convergence,  as  one  of  the  three  factors 
which  contribute  to  comfortable  vision  at  the  near  point. 

2d.  In  ordinary  near  work  which  requires  the  individual 
to  look  down  at  an  angle  of  35  or  40  degrees,  and  to  con- 
verge at  a  point  about  30  centimeters  distant,  almost  the 
minimum  amount  of  torsion  is  ordinarily  demanded. 

3d.  Even  a  slight  disturbance  of  the  normal  amount  of 
torsion  may  be  a  source  of  discomfort,  and  this  happens  in 
some  cases,  at  least,  when  an  astigmatic  glass  is  not  set  at 
the  proper  angle.  That  will  be  considered  in  detail  in  the 
part  of  our  study  which  relates  to  pathologya 


CHAPTER  IX. 

BALANCE  OF  THE  OCULAR  MUSCLES. 

§  i.  Factors  in  the  Production  of  Comfortable  Vis- 
ion at  the  Near  Point. — We  have  been  devoting  our- 
selves principally  to  the  study  of  three  functions  of  the 
ocular  muscles,  namely,  accommodation,  convergence,  and 
torsion.  Each  one  of  these  has  been  studied  separately  and 
also  in  its  relation  to  the  other  two  functions.  But  we  must 
not  lose  sight  of  the  equally  important  fact  that  each  one  of 
them  is  directly  influenced  by  what  may  be  called  the  "re- 
sistance" to  the  muscular  effort  made.  It  was  because  of  the 
importance  of  this  resistance  offered, that  several  apparent  di- 
gressions have  already  been  made  in  the  course  of  our  study. 
Thus  taking  the  act  of  accommodation  we  have  concluded 
that  the  principal  function  of  the  intraocular  muscles  is  to 
change  the  form  of  the  lens  and  the  size  of  the  pupil  so  as 
to  produce  a  clear  focus  on  the  retina.  But  the  effort  to 
accomplish  that  result  may  be  made  difficult  or  impossible 
by  what  may  be  called  the  "resistance"  offered.  This 
may  be,  as  we  know,  an  excessive  effort  which  must  be 
made  either  because  the  eye  is  too  short,  or  because  the 
lens  has  become  somewhat  hardened,  or  because  the  cornea 
or  the  lens,  or  both,  have  an  irregular  curvature,  or  even  be- 
cause the  innervation  of  the  ciliary  muscle  is  insufficient. 
Any  or  all  of  these  conditions  may  constitute  what  we  may 
call  resistance.  For  the  sake  of  convenience,  we  may  indi- 
cate this  resistance  to  the  accommodation  by  Rj. 

In  the  same  way  convergence  may  be  influenced  more  or 
less  by  the  resistance  offered  to  it.  That  may  be  because 
of  the  excessive  development  of  any  one  or  all  of  the 
abductor  group  of  muscles,  or  imperfect  development  of 
any  one  of  the  adductor  group,  or  the  obstacle  to  be  over- 

366 


Diagram  of  Muscle  Action  367 

come  may  depend  on  the  shape  of  the  globe,  as  in  myopia, 
or  spring  from  imperfect  innervation  of  the  adductors,  or 
even  be  due  indirectly  to  insufficient  action  of  the  ciliary 
muscle.  Any  or  all  of  these  conditions,  or  conditions  simi- 
lar to  them,  we  may  group  together  under  the  general  term 
of  resistance  to  the  convergence,  and  indicate  it  by  R2. 

It  would  appear  that  the  same  principles  apply  to  torsion, 
as  far  as  it  is  possible  to  make  any  statement  concerning 
that  third  factor.  When  one  group  of  muscles  is  called 
into  action  in  rotating  the  upper  end  of  the  vertical  axes 
outward,  it  is  fair  to  say  that  the  opponents  of  that  group 
act  as  a  resisting  power  in  preventing  an  insufficient  or  an 
excessive  torsion.  Indeed,  clinical  evidence  shows  beyond 
doubt  that  the  power  of  making  sufficient  torsion  is  in- 
fluenced by  pathological  conditions.  That  obstacle  or  ob- 
stacles, whichever  it  may  be  considered,  produces  the 
resistance  offered  to  the  muscles  which  normally  produce 
torsion,  and  this  resistance  we  may  indicate  by  R3.  Evi- 
dently, therefore,  in  considering  comfortable  vision  for  the 
near  point,  we  have  to  do  with  the  three  primary  factors, 
accommodation,  convergence,  and  torsion,  and  in  addition 
we  must  consider  the  resistance  which  is  offered  to  each 
of  them.  That  resistance  may  be  a  secondary  factor,  it 
is  true,  but  nevertheless  it  must  be  taken  into  account. 
Practically,  therefore,  we  have  three  pairs  of  factors,  or 
at  least  six  different  elements,  as  it  were,  in  the  act  which 
we  call  vision  at  a  near  point. 

§  2.  A  Simple  Method  of  Representing  by  Diagram 
the  Amount  of  Accommodation,Convergence,  or  Torsion 
Which  Exists  in  a  Given  Case  Together  with  the 
Amount  of  Resistance  Offered.— This  will  be  understood 
at  once  by  recalling  the  well-known  method  of  representing 
accommodation  by  a  straight  line.  It  is  seen  in  A,  Fig. 
228.  It  is  an  old  method  (page  144)  often  employed  by 
Landolt  and  others,  and  is  simply  a  graphic  representation 
of  the  amount  of  accommodation. 

If  ophthalmoscopic  examination  and  other  tests  show  the 
eye  is  emmetropic,  and  the  resistance  to  the  accommodation 
is  normal — that  is,  in  proportion  throughout  to  the  amount 


368 


Diagram  of  Muscle  Action 


of  accommodation,  —  we  can  represent  the  resistance  of 
the  emmetrope  on  a  similar  horizontal  line,  which  is  also 
divided  into  equal  parts,  each  representing  one  diopter.  It 
is  evident  that  the  line  which  represents  this  would  be  of 
the  same  length  and  would  extend  through  the  same  divis- 
ions as  does  the  line  which  represents  the  range  of  accom- 
modation. This  is  shown  by  the  line  Rj. 

Just  as  the  range  of  accommodation  can  be  represented 
on  a  line  divided  into  equal  parts,  so  we  can  represent  the 
range  of  convergence  on  a  line  divided  into  equal  parts,  of 
the  same  size  as  those  which  represent  accommodation, 
each  of  which  divisions  corresponds  to  one  meter  angle. 
Thus,  if  the  range  extends  from  infinity  to  ten  meter  angles, 


13  12   I  MO    9    5     7    6    5    4    3 


oo  |     234 


A 

Ri 

C 

R2 

T 
P3 

FIG.  228. — Diagrammatic  illustration  of  muscle  balance  (Eukinesis). 
In  this  young  person  the  accommodation,  convergence,  and  torsion,  with  the 
resistance  to  each,  is  normal  up  to  10  D. 

such  a  range  would  evidently  occupy  ten  of  the  spaces  re- 
ferred to. 

Also,  the  resistance  offered  to  the  convergence  may  be 
represented  on  a  similar  line  divided  into  equal  parts,  each 
of  which  corresponds  to  one  meter  angle  of  convergence. 
This  is  seen  in  R2.  The  method  of  estimating  the  amount 
of  resistance  which  is  offered  to  convergence  will  be  elab- 
orated later.  At  this  point,  it  must  suffice  to  say  that  it 
depends  upon  whether  the  individual  under  examination 
shows  an  orthophoria,  an  esophoria,  or  exophoria,  and  if 
either  of  the  latter  is  present,  then  in  what  degree.  Thus, 
with  orthophoria  we  may  assume  that  if  the  eyes  are  other- 
wise normal,  then  the  power  of  convergence  is  in  proportion 
to  the  amount  of  accommodation  exerted.  When  exo- 


Muscle  Balance  369 

phoria  exists,  and  in  a  degree,  for  example,  representing  two 
meter  angles,  the  adductors  have  that  extra  amount  to 
overcome. 

Esophoria  may  be  shown  in  a  similar  manner.  Finally, 
torsion  and  the  resistance  to  it  can  each  also  be  represented 
by  divisions  on  a  line.  As  we  know  from  the  tables  of 
Landolt  and  from  the  other  measurements  of  normal 
eyes  about  how  many  units  of  torsion  in  the  horizontal 
plane,  for  example,  correspond  to  a  given  amount  of  con- 
vergence, so  it  is  possible  to  represent  torsion  on  a  line 
divided  into  equal  parts,  each  one  of  which  corresponds  to 
one  diopter  of  accommodation.  (T,  Fig.  228.) 

The  resistance  which  is  offered  to  torsion  with  a  given  de- 
gree of  convergence  could  also  be  represented  on  a  similar 
line  and  in  a  similar  manner.  It  is  true  that  our  know- 
ledge  of  this  latter  subject  is  as  yet  far  from  complete,  but 
such  facts  as  we  have  warrant  us  at  least  in  recording 
what  we  do  know  of  torsion,  just  as  we  record  both  accom- 
modation and  convergence,  and  the  resistance  offered  to 
each  of  them. 

It  would  lead  to  too  long  a  digression  at  this  point  to 
describe  how  it  is  possible  to  represent  in  this  way  ab- 
normal accommodation,  convergence,  or  torsion,  and  the 
resistance  which  is  offered  to  each,  but  in  the  future  it  will 
be  necessary  to  refer  quite  in  detail  to  these  graphic  repre- 
sentations of  muscle  imbalance.  The  object  in  calling 
attention  to  this  graphic  method  at  this  point  is  merely  that 
we  may  obtain  a  clearer  mental  image  of  each  of  these  six 
different  factors  and  their  relation  to  each  other. 

§  3.  Muscle  Balance.— This  term  muscle  balance  has 
been  used  to  express  almost  anything  or  nothing.  It  is 
too  often  confused  with  what  Stevens  called  orthophoria 
(B  725)  or  with  his  euphoria  (B  543,  p.  227).  In  the  very 
excellent  nomenclature  which  he  proposed  and  which  we 
have  generally  adopted,  the  'phorias,  as  repeatedly  stated, 
refer  to  passive  tendencies  of  the  eyes  to  assume  certain 
positions.  They  do  not  take  into  account  in  any  way  the 
action  of  the  ciliary  muscles.  But  when  we  speak  of 

24 


37o  Muscle  Balance 

muscle  balance  we  refer  not  simply  to  the  extraocular 
but  also  to  the  intraocular  muscles.  In  view  of  this,  and 
of  the  foregoing,  we  may  say  that  muscle  balance  is  tJie 
condition  in  which  with  comfortable  binocular  vision  accommo- 
dation, convergence,  and  torsion  bear  tlieir  normal  relations 
to  each  other.  Such  a  definition,  although  elastic,  is  better 
than  none,  for  we  evidently  need  a  physiological  standard 
with  which  to  compare  various  forms  of  muscle  imbalance, 
or  unbalance,  as  it  is  sometimes  called. 

From  this  definition  it  is  evident  that  the  term  muscle 
balance  is  relative  and  depends  on  numerous  conditions.  A 
few  of  these  should  be  mentioned,  for  example: 

(A)  The  age  of  the  individual  influences  greatly  what 
may  be  called  the  range  of  muscle  balance.     Thus  in  early 
youth,  when  the  range  of  accommodation  and  convergence 
is  large,  there  is  a  corresponding  large  range  in  which  we 
can  expect  to  find  muscle  balance. 

(B)  The  employment  of  the  individual  affects  muscle 
balance.     Persons    who    use   their  eyes   only    occasionally 
for  close  work  can   maintain  that   effort   for  a  longer  time 
each  day  than  do  those  who  must  tax  the  ocular  muscles  by 
constant  near  work. 

(C)  The  so-called  general  health  of  the  individual  is  a 
factor  of  no  small  importance  in  the  maintenance  of  muscle 
balance. 

(D)  All  such  factors  vary  with  the  same  individual  at 
different  times.    Thus,  a  person  may  have  perfect  muscle 
balance   when  looking   at  a  distant   object,  but  not  at  20 
cm.  or  25  cm.  or  even  at  33  cm ;  or  he  may  have  muscle 
imbalance  without  glasses  and  muscle  balance  with  them.   In- 
deed, we  shall  learn  that  the  principal  object  of  glasses  is  to 
make  the  balance  as  perfect    as   possible   when    an  imbal- 
ance  exists.       The   English    term    muscle    balance  is   evi- 
dently better  than    any    other,    though    if  we   wish  for  a 
synonym    of   classic    terminology  we  could   use  eukinesis 
(eu,  well ;  and  kinesis,    strength). 

The  differences  between  orthophoria  and  muscle  balance  or 
eukinesis  can  be  understood  best  by  comparing  one  with 
the  other,  thus: 


Muscle  Balance 


Orthophoria 

1.  Relates    to     extrinsic 

muscles  only. 

2.  Visual    lines    tend  to 

parallelism. 

3.  Binocular  vision    may 

or  may  not  exist. 

4.  Comfort   may  or  may 

not  exist. 


Muscle  Balance  (Eukinesis). 

1.  Relates  to  extrinsic  and 

intrinsic  muscles. 

2.  Visual  lines  parallel  or 

converging. 

3.  Binocular  vision   must 

exist. 

4.  Comfort  is  essential. 


CHAPTER  X. 

RELATION  OF  THE  "  GENERAL  STRENGTH  "   TO  THE  PHYSI- 
OLOGICAL ACTIONS   OF  THE   OCULAR   MUSCLES. 

We  have  now  completed  a  review  of  the  functions  of  the 
various  muscles  of  the  eye.  But  some  mention  should  be 
made  of  the  relation  between  the  fusion  power  or"  strength  " 
of  the  ocular  muscles  and  what  may  be  termed  the  general 
strength  of  the  individual.  In  dealing  with  this  question  it  is 
desirable  to  express  the  relationship  in  figures  as  far  as  pos- 
sible— that  is,  to  obtain  the  power  of  adduction,  abduction, 
etc.,  in  terms  of  a  prism,  then  to  determine  the  condi- 
tion of  the  muscles  of  the  body  as  expressed  in  foot- 
pounds, and  finally  to  compare  these  two  with  each  other. 

We  have  already  learned  howthestrength  of  the  eye  muscles 
can  be  expressed  approximately,  at  least,  in  terms  of  a  prism. 
It  is  sufficient,  therefore,  at  this  point  to  observe  that  when 
we  take  into  account  both  the  minimum  and  maximum  power 
of  different  groups  of  the  ocular  muscles — for  example,  ad- 
duction and  abduction — each  measured  thus  by  different 
methods,  we  have  at  least  an  approximate  expression  of 
the  strength  of  the  ocular  muscles  in  that  individual. 

Second,  let  us  see  what  is  meant  by  the  term  general 
muscular  strength  or  strength  of  other  muscles,  and  ascertain 
how  that  also  can  be  expressed  in  figures.  Formerly  it  was 
supposed  that  this  might  be  estimated  by  the  weight  of  a 
dumb-bell  which  could  be  lifted,  or  by  the  performance  of 
some  other  special  feat.  But  in  these  tests  much  depends 
on  a  single  group  of  muscles  or  on  some  art  of  the  per- 
former. Several  forms  of  dynamometers  have  also  been 
constructed,  some  of  which  are  excellent,  but  they  show 
only  the  strength  in  the  arms  as  the  instrument  is  grasped 

372 


Ocular  Muscles  and  Other  Muscles  373 

forcibly  by  the  hands,  or  of  the  arms  and  legs,  etc.  Of 
late  years  rather  more  accurate  measurements  of  the  gen- 
eral muscular  strength  have  been  secured,  by  Dr.  Sar- 
gent, Director  of  the  Hemenway  Gymnasium  at  Harvard. 
He  evolved  what  he  calls  a  "  Universal  Test  for  Strength, 

o         ' 

Speed,  and  Endurance"  (8825),  which  is  simple,  and  is 
destined  apparently  to  be  of  some  value  to  ophthalmologists 
as  well  as  to  physicians,  because  this  shows  what  the 
general  muscular  condition  of  an  individual  is  at  any  given 
time,  and  because  the  exercises  which  serve  as  this  "  test  of 
strength  "  constitute  also  an  excellent  method  of  improving 
the  muscular  tone. 

It  would  necessitate  too  long  a  digression  at  this  point  to 
describe  each  of  these  tests  of  strength  in  detail.  They  will 
be  discussed  in  the  chapter  in  the  second  volume  which 
deals  with  central  asthenopia.  For  our  present  purpose  it 
must  suffice  to  say  that  these  exercises  consist  simply  in 
lifting  certain  parts  of  the  body  a  certain  number  of  times 
within  a  certain  number  of  minutes.  Thus,  if  an  individual 
who  weighs  1 50  pounds,  and  who  is  five  feet  high,  lying  on  his 
back  can  lift  himself  to  a  sitting  posture  (approximately  75 
pounds  two  and  a  half  feet),  and  do  this  thirty  times  within  ten 
minutes,  evidently  the  strength  expended  can  be  stated  as 
5,625  foot-pounds.  Various  other  exercises  of  a  similar  kind 
can  be  used  which  give  us  a  standard  for  measuring  the  gen- 
eral strength  of  that  individual.  It  has  been  found  in  gen- 
eral that  the  average  healthy  man  can  lift  in  this  way  in 
half  an  hour  about  forty-five  to  fifty  thousand  foot-pounds, 
although  by  systematic  exercise  the  amount  may  be  in- 
creased, as  with  athletes,  to  seventy  or  eighty  thousand. 
The  general  strength  of  women  is  naturally  less,  and  ranges 
from  about  twenty  or  thirty  to  forty  thousand  foot-pounds 
in  half  an  hour.  In  children  it  is,  of  course,  proportionately 
less. 

The  next  question  before  us  is  whether  any  relation 
exists  between  the  strength  of  the  ocular  muscles  as  ex- 
pressed in  terms  of  a  prism,  and  the  general  strength  of  the 
individual  as  expressed  thus  in  foot-pounds.  To  obtain  some 
idea  of  this,  an  examination  was  made  by  Dr.  Charles  H. 


374  Ocular  Muscles  and  Other  Muscles 

Williams  of  Boston  and  myself,  at  the  Harvard  Gymnasium, 
of  the  normal  eyes  of  twenty-nine  students  whose  general 
strength  had  been  carefully  measured.  Also,  in  a  consider- 
able number  of  cases  of  heterophoria  in  which  for  various 
reasons  systematic  muscle  exercise  was  practised,  the  general 
strength  has  also  been  recorded. 

The  conclusions  from  these  observations,  briefly  stated, 
are: 

First :  Suitable  tests  show  that  under  normal  conditions 
the  minimum  power  of  adduction  and  abduction  remains 
at  about  the  normal  amount,  no  matter  what  may  be  the 
general  strength  of  the  individual. 

Second:  The  maximum  power  of  adduction  is  to  a  cer- 
tain extent  in  proportion  to  the  general  strength  of  the 
individual. 

Third :  In  cases  of  heterophoria,  when  so-called  muscle 
exercise  is  practised  when  the  general  strength  of  the  in- 
dividual is  quite  up  to  the  normal  standard,  the  maximum 
power  of  adduction  or  abduction  can  be  increased  more 
rapidly,  on  the  average,  than  in  other  individuals  whose 
general  strength  is  apparently  less  than  normal. 

In  any  such  discussion  it  would  be  an  omission  not  to 
take  into  account  what  may  be  called  the  "  muscle  tone." 
This  condition,  long  recognized  by  physiologists  as  the 
tonus  muscularis,  does  not  refer  to  the  strength  or  lifting 
power.  It  is  not  easily  defined,  though  well  recognized  as 
the  ability  of  a  muscle  or  group  of  muscles  to  perform  the 
amount  of  work  normally  devolving  upon  them  without  the 
development  of  fatigue  in  those  muscles  or  in  other  parts 
of  the  body.  Unfortunately  we  have  no  exact  methods  of 
measuring  this  tone  except  by  the  feelings  of  the  individual, 
and  these,  of  course,  are  much  influenced  by  the  personal 
equation.  There  can  be  no  question,  though,  but  that  this 
normal  tone  of  the  ocular  muscles  is  also  in  proportion  to 
the  tone  of  the  muscles  in  the  other  parts  of  the  body  of 
that  individual.  Thus,  as  a  rule,  we  find  that  where  the 
muscle  tone  of  the  individual  is  low— that  is,  where  the  other 
muscles  are  easily  fatigued  or  utterly  unable  to  do  their 
work,  there  is  usually  a  difficulty  of  the  ocular  muscles  also 


Ocular  Muscles  and  Other  Muscles  375 

in  performing  the  work  which  is  imposed  upon  them.  On 
the  other  hand,  when  the  general  muscle  tone  is  high,  as 
we  find  in  a  strong  and  well  developed  individual,  the  tone 
of  the  ocular  muscles  bears  a  certain  relation  to  that.  It  is 
true  that  we  often  find  invalids  who  can  read  and  write 
all  day  under  adverse  circumstances  without  inconvenience, 
but  these  are  rather  exceptions  to  the  general  rule. 

The  practical  importance  of  this  relation  between  the 
strength  or  the  tone  of  the  ocular  muscles  and  the  strength 
or  tone  of  the  muscles  of  the  body  as  a  whole  is  apparent  at 
once.  It  means,  in  a  word,  that  where  we  find  an  imbalance 
or  fault  of  the  muscles,  we  should  not  be  satisfied  in  attempt- 
ing to  correct  this  by  optical  appliances  or  other  local  means, 
but  whenever  the  general  strength  of  the  individual  is  at  all 
below  the  normal  standard,  as  measured  by  exact  tests,  the 
strength  or  tone  of  the  other  muscles  of  the  body  should 
also  be  improved. 


CHAPTER  XI. 

RECAPITULATION  AND   CONCLUSIONS. 

§  i.  Recapitulation. — When  concluding  one  of  his  courses 
of  lectures,  Tyndall  compares  the  progress  made  by  the  stu- 
dent to  the  gradual  ascent  of  a  mountain,  and  reviews  the 
course  pursued  in  order  to  obtain  a  general  view  of  the 
entire  subject.  So  can  we  also  now  recapitulate  with  advan- 
tage the  salient  points  observed,  and  estimate  what  real  pro- 
gress, if  any,  has  been  made,  and  what  conclusions  we  have 
reached  in  this  study  of  the  muscles  of  the  eye. 

We  began  in  the  most  elementary  manner  with  dissection 
of  the  extraocular  muscles ;  we  observed  the  difference  be- 
tween their  primary  and  secondary  insertions,  particular 
attention  being  given  to  the  latter,  which,  though  so  otten 
disregarded,  are  none  the  less  of  extreme  importance  in  their 
relation  to  operations  for  tenotomy  and  advancement. 

We  examined  next  the  intraocular  muscles,  or  rather  the 
different  structures  concerned  in  accommodation.  This 
meant  not  simply  the  ciliary  muscle,  and  the  manner  in 
which  it  was  connected  with  the  lens,  but  the  position  and 
structure  of  the  lens  itself.  In  doing  this  we  made  a  modi- 
fication of  the  Javal  ophthalmometer  to  show  the  position 
of  the  lens.  For,  as  the  object  of  the  ciliary  muscle  is  to 
change  the  form  of  the  lens  so  as  to  produce  a  perfect  focus, 
imperfections  in  the  position  or  structure  of  the  lens,  or  im- 
perfections in  other  medias,  present  what  might  be  considered 
a  resistance  to  the  action  of  that  muscle.  These  different  im- 
perfections, as  they  occur  in  the  practically  normal  eye,  were 
therefore  examined  with  some  detail.  Moreover,  in  passing, 
we  noted  also  the  relation  which  the  muscles  of  the  fore- 
head, especially  the  two  parts  of  the  occipito-frontalis,  bear 
to  the  act  of  accommodation  ;  for,  as  we  shall  see  later,  the 

376 


Recapitulation  377 

contraction  of  these  muscles  in  abnormal  accommodation  is 
at  least  one  of  the  sources  of  "  ocular  headaches." 

In  the  study  of  the  nerve  supply  of  the  muscles  we  again 
began  with  the  most  elementary  facts,  and,  adding  to  these 
the  latest  discoveries  of  the  best  observers,  tried  to  obtain  a 
clear  picture  of  the  various  nuclei,  their  structure,  and  the 
distribution  of  the  different  fibers.  This  part  was  neces- 
sarily a  digest  of  the  work  of  well-known  histologists  and 
physiologists. 

Comparative  anatomy  and  embryology  were  examined 
only  briefly,  in  spite  of  the  temptation  to  dwell  upon  them 
because  of  their  general  scientific  interest. 

Passing  next  to  the  physiology  of  the  muscles,  we  con- 
sidered first  one  eye  at  rest,  in  order  that  we  might  review 
those  fundamental  principles  relating  to  it,  as  a  globe  ro- 
tated in  different  directions  by  the  different  muscles.  In 
this  connection  we  found  that  by  modifying  the  ordinary 
Javal-Schiotz  ophthalmometer  it  was  possible  to  determine 
with  that,  quite  exactly,  the  center  of  motion. 

A  special  chapter  was  also  devoted  to  the  globe  in  action, 
but  not  necessarily  in  motion,  as  when  the  internal  muscles 
contract  in  accommodation.  For  as  that  act  must  be  con- 
stantly referred  to,  especially  when  studying  the  pathology 
of  the  muscles,  it  seemed  essential  to  understand  just  what 
is  meant  by  it.  In  that  connection  we  glanced  at  the  well- 
known  action  of  full  doses  of  cycloplegics  and  of  mydriatics, 
and,  what  was  quite  as  important,  we  observed  the  effect  of 
what  we  called  "  minimum  doses"  of  several  of  these  drugs 
upon  the  normal  eye.  This  gave  us  a  physiological  standard, 
which,  within  certain  limits  at  least,  serves  as  a  measure  by 
which  to  determine,  in  any  given  case,  whether  the  ciliary 
muscle  relaxes  or  contracts  more  promptly  than  natural.  In 
other  words,  this  helps  us  to  determine  whether  there  exists 
a  tendency  to  excessive  or  to  insufficient  accommodation. 

In  considering  the  manner  in  which  the  extraocular 
muscles  move  the  globe,  it  seemed  best  to  confine  our  atten- 
tion first  to  the  motion  of  one  eye  only.  In  this  way  we 
could  study  a  large  part  of  the  ocular  motions,  eliminating 
all  forms  of  associated  movements.  For  that  purpose  we 


378  Recapitulation 

constructed  a  new  ophthalmotrope.  We  observed  what  the 
action  is  of  a  single  muscle.  We  noted  for  the  first  time  the 
lifting  power  of  the  adductors  and  the  tensile  strength  of 
the  recti.  We  simplified  the  apparatus  to  measure  the 
rapidity  of  the  lateral  movements  of  the  eyes  by  means  of 
photography.  We  found  what  that  motion  is  in  the  natural 
condition,  thus  obtaining  a  standard  by  which  to  determine 
in  doubtful  cases-  whether  there  is  an  imperfection  of  the 
movement  to  one  side  or  the  other.  Also,  we  examined  at 
considerable  length  the  limits  of  the  field  of  fixation ;  we 
studied  the  claims  of  the  tropometer  and  of  the  perimeter 
to  exactness  in  measurements  of  this  kind,  and  found  that 
various  improvements  could  be  made,  especially  in  the  latter 
instrument. 

As  one  eye  was  studied  first  at  rest  and  then  in  motion, 
so  the  two  eyes  acting  together  were  considered  first  at 
rest  and  then  in  motion.  We  found  it  was  by  no  means 
easy  to  ascertain  just  what  position  the  eyes  assume  when  in 
a  position  of  "  rest,"  and  the  different  tests  of  the  static 
position  were  therefore  divided  into  groups  in  order  to  dis- 
tinguish them  more  readily  from  each  other.  Thus,  we  had 
first  a  group  of  tests  which  produced  a  displacement  of  one 
or  both  retinal  images,  a  second  group  including  those  which 
produced  a  blurring  of  one  of  the  retinal  images,  while  the 
third  group  included  those  in  which  one  eye  was  covered  or 
excluded  from  the  act  of  binocular  vision.  We  compared 
these  different  groups  of  tests  with  each  other,  to  ascertain 
as  nearly  as  possible  their  relative  values.  We  found  that 
orthophoria  was  by  no  means  always  present  in  the  normal 
eyes,  but  that  the  majority  of  practically  perfect  non- 
asthenopic  eyes  tend  to  turn  sometimes  in  one  direction, 
sometimes  in  another,  in  order  to  assume  their  position  of 
rest. 

After  understanding  the  position  which  the  eyes  tend  to 
assume  when  "  at  rest,"  we  were  better  prepared  to  examine 
the  motions  which  the  two  eyes  make  in  an  effort  at  bin- 
ocular vision.  These  associated  movements  were  found  to 
separate  themselves  also  into  certain  groups. 

Thus,  in  the  first  group  of  motions,  the  visual  axis  being 


Recapitulation  379 

in  the  primary  position,  the  vertical  axes  turn  in  or  out. 
This  led  us  to  glance  at  various  earlier  methods  of  measur- 
ing the  true  torsion  or  wheel  motion  which  is  possible  with 
parallel  visual  axes.  We  examined  the  Volkmann  discs, 
the  application  of  them  to  the  Stevens  clinoscope,  and  then 
applied  them  to  a  new  form  of  the  clinoscope  apparently 
simpler  and  with  a  wider  range  of  usefulness. 

A  second  group  of  motions  we  found  to  include  those  in 
which  the  parallel  visual  axes  move  in  some  one  of  the 
principal  meridians,  up,  down,  in,  and  out.  By  the  use  of 
after-images  we  found  that  in  these  motions  no  true  torsion 
occurred. 

The  third  group  of  associated  movements  we  learned  in- 
cluded those  in  which  the  visual  axes  moved  obliquely  from 
the  primary  position  into  some  secondary  position.  In 
doing  this  again  no  true  torsion  occurred,  but  there  did 
occur  a  certain  apparent  turning  of  the  vertical  axes  about 
the  visual  axes  in  such  a  way  as  to  give  to  the  obse.rver  of  the 
after-images  the  effect  of  torsion.  This  led  us  to  the  calcula- 
tion of  the  degree  of  false  torsion  by  a  new  formula. 

The  .fourth  group  of  associated  movements  we  found  to 
be  the  most  important  in  its  clinical  aspects ;  so  important, 
indeed,  that  in  order  to  study  it  properly  it  was  considered 
in  five  divisions.  As  a  preliminary  step,  we  reviewed 
the  well-known  facts  relating  to  ophthalmological  prisms — 
we  glanced  at  the  meaning  of  the  meter  angle,  expressed 
the  size  of  meter  angles  in  degrees  with  exactness,  and  con- 
structed a  table  of  degrees  in  terms  of  the  meter  angle.  We 
also  established  the  difference  between  the  minimum  and 
maximum  fusion  power  —  a  point  of  no  small  clinical 
importance. 

Thus  we  were  better  prepared  to  study  the  variations  in 
accommodation  with  a  given  degree  of  convergence — that  is, 
relative  accommodation.  We  saw  how  it  could  be  illustrated 
graphically;  we  ascertained  how  the  measurement  of  rela- 
tive accommodation  was  made,  and  the  results  plotted,  and 
observed  how  relative  accommodation  is  influenced  by  in- 
creasing  age, — this  last  fact  being  of  decided  clinical  value  in 
determining  whether  or  not  the  ciliary  muscles  have  a  normal, 


380  Conclusions 

a  subnormal,  or  an  excessive  power  of  contraction.  After 
studying  relative  accommodation  we  passed  to  relative  con- 
vergence, saw  what  that  was,  what  the  desiderata  were  for 
its  measurements,  and  also  what  degree  of  exactness  was 
necessary  for  clinical  purposes. 

Finally  we  examined  the  true  torsion  which  accompanies 
convergence,  and  saw  what  this  is  in  the  horizontal  plane 
and  in  other  planes  inclined  below  or  above  the  horizontal. 

Also  what  we  may  call  relative  torsion  was  treated  as  we 
had  already  treated  relative  accommodation  and  relative 
convergence.  We  saw,  however,  that  these  complete 
measurements  are  required  for  clinical  purposes  only  in 
rather  unusual  cases,  and  that  ordinarily  a  very  simple  pro- 
cedure is  sufficient  to  indicate,  at  least,  whether  the  amount 
of  relative  accommodation,  or  convergence,  or  torsion  pos- 
sessed by  a  given  individual  can  be  considered  normal  or 
abnormal. 

The  facts  ascertained  up  to  that  point  led  us  to  the  con- 
clusion that  in  the  act  of  comfortable  vision  for  the  near 
point  we  have  to  deal  with  three  principal  factors — namely, 
accommodation,  convergence,  and  torsion  ;  or,  if  we  take 
into  account  what  may  be  called  the  resistance  to  each  of 
these,  that  we  have  then  three  secondary  factors,  or  six 
altogether.  In  considering  these  different  factors  we  found 
that  as  long  as  they  all  acted  together,  within  normal  limits, 
there  existed  a  condition  which  might  be  called  muscle 
balance. 

§  2.  Conclusions. — After  this  study  of  the  anatomy  and 
physiology  of  the  ocular  muscles,  and  after  a  recapitulation 
of  the  points  to  which  attention  has  been  specially  called, 
still  the  practitioner  may  ask  himself — What  of  it  ?  How 
do  these  tests  and  methods  of  examination  lessen  our  con- 
fusion of  ideas  concerning  the  ocular  muscles,  or  assist  us  in 
the  routine  of  daily  work  ?  This  is  a  natural  question,  and 
even  such  a  partial  answer  as  can  be  given  while  our  study 
of  the  subject  is  only  half  finished  may  be  better  than  none 
at  all.  In  a  general  way,  therefore,  it  would  seem  that  thus 
far  we  are  led  to  certain  conclusions : 

The  first  one  is  that  a  thorough  study  of  the  anatomy 


Conclusions  381 

and  physiology  of  the  muscles  is  essential  to  a  working 
knowledge  of  their  pathology.  At  first  glance  it  seems 
a  waste  of  words  to  state  such  an  axiomatic  truth  as  a 
conclusion.  But  when  we  consider  the  complexity  of  the 
questions  involved  in  this  subject,  how  they  have  to 
do  with  mechanics  and  optics — both  involving  the  higher 
mathematics, — also,  how  a  study  of  the  fusion  of  images 
leads  us  into  the  borderland  between  physiology  and  psy- 
chology,— when  these  and  other  details  are  considered,  we 
may  even  say  that  the  completeness  of  our  clinical  work 
with  the  muscles  is  about  in  proportion  to  our  knowledge 
of  their  anatomy  and  physiology.  It  is  true  that  some  of 
the  facts  stated  in  the  foregoing  pages  are  given  rather  for 
completeness,  and  many  others  can  be  found  in  the  litera- 
ture which  are  unnecessary  refinements  made  by  laboratory 
students  and  have  no  bearing  on  clinical  work.  But  it  is 
also  quite  true  that  the  knowledge  which  the  average  oph- 
thalmologist has  of  the  anatomy  and  physiology  of  the 
muscles  of  the  eye  is  not  sufficient  for  the  best  clinical 
work. 

Second,  more  exact  definitions  are  necessary  of  much  of  the 
anatomy  and  physiology  of  the  ocular  muscles  if  we  would 
clear  up  in  any  way  our  confusion  of  ideas  concerning  them. 
For  example,  from  the  anatomical  standpoint,  and,  there- 
fore, for  surgical  purposes,  we  must  distinguish  the  primary 
from  the  secondary  insertions.  Or  physiologically  we  must 
distinguish  the  passive  states  (the  'phorias)  from  the  active 
conditions  (the  'ductions).  Among  the  latter  we  must  also 
distinguish  the  minimum  from  the  maximum  power  of  ad- 
duction, abduction,  etc.  By  keeping  these  and  many  other 
such  differences  in  mind  our  ideas  of  the  subject  become 
proportionately  clarified. 

Third,  we  need  greater  uniformity  in  methods  of  ex- 
amination. No  matter  whether  a  test  is  made  of  the  static 
or  dynamic  condition,  or  whether  measurements  are  made 
of  the  power  of  accommodation  or  of  convergence  or  of 
torsion,  or  of  any  one  of  these  with  reference  to  the  other, 
we  should  agree  on  certain  procedures  to  be  adopted ;  or, 
if  that  cannot  be  done,  then  each  ophthalmologist  should 


382  Conclusions 

describe  his  methods  in  his  own  records  for  his  own  con- 
venience, or  surely  when  writing  for  others.  The  results 
which  we  obtain  depend  to  so  great  a  degree  upon  the  de- 
tails of  examination,  that  uniformity  is  a  necessity  if  we 
would  understand  each  other,  or  if  the  same  observer  wishes 
to  interpret  intelligently  observations  made  by  himself  at 
different  times. 

Fourth,  as  a  corollary  from  the  foregoing,  we  may  conclude 
that  the  practitioner  must  make  his  clinical  examinations 
much  more  thorough  than  is  usual,  if  he  wishes  to  obtain  a 
sufficient  number  of  data  upon  which  to  base  a  final  opinion. 

With  most  ophthalmologists  the  routine  of  examination 
is  about  as  follows :  To  obtain  the  clinical  history,  measure 
the  refraction  first  objectively,  then  subjectively,  the  range 
of  accommodation,  the  relative  accommodation  at  the  far 
point,  and  perhaps  at  the  near  point.  As  for  the  extrinsic 
muscles,  the  static  condition  with  parallel  axes  is  tested,  and 
perhaps  at  three  meter  angles  of  convergence.  Also  the 
dynamic  condition  for  distance  is  measured,  and  occasion- 
ally at  the  near  point.  In  cases  where  the  surgeon  happens 
to  judge  that  a  cycloplegic  is  indicated,  he  drops  some 
homatropin  or  atropin  in  the  eye,  not  knowing  how  much 
is  used,  nor  caring  for  any  effect  except  to  place  the  accommo- 
dation at  rest,  if  indeed  that  is  done. 

Such  an  outline,  or  one  which  is  much  more  schematic  and 
scanty,  furnishes  all  the  data  which  it  is  usually  deemed 
necessary  to  have.  Many  practitioners  are  satisfied  with  a 
much  less  thorough  examination  than  this,  certainly  at  the 
first  consultation,  and  some  do  not  make  it  more  complete 
no  matter  how  often  an  opportunity  for  re-examination  is 
presented. 

Although  this  outline,  or  one  similar  to  it,  is  sufficient  in 
a  certain  way,  and  although  it  is  often  more  than  is  possible 
for  a  busy  practitioner  in  his  daily  work,  yet  any  reader  of 
these  pages  must  admit  to  himself,  if  not  to  others,  that  a 
diagnosis  based  upon  such  data  must  necessarily  be  only  a 
provisional  one. 

The  truth  is  that  a  certain  number  of  cases  do  return 
to  us  with  the  same  symptoms  in  spite  of  all  the  care  that 


Conclusions  383 

can  be  given  at  an  ordinary  first  visit.  In  fact,  they  re- 
turn not  only  once,  but  a  second  or  third  time,  or 
perhaps  many  times,  still  with  the  same  complaints; 
or  else,  being  discouraged  and  dissatisfied  with  one 
practitioner,  they  float  about  from  office  to  office,  each 
time  passing  through  the  same  routine  of  superficial 
examination. 

Now  one  of  the  main  objects  of  this  entire  study  is  to 
show  that  the  practitioner  may  still  learn  a  great  deal  more 
concerning  these  unsatisfactory  cases,  if  he  makes  his  ex- 
aminations according  to  the  methods  which  have  here  been 
even  imperfectly  outlined. 

In  order  to  see  more  specifically  what  this  means,  let  us 
suppose,  first,  that  we  have  to  do  with  a  case  in  which  the 
person  complains  of  the  cardinal  symptoms  of  what  we 
usually  call  asthenopia.  If  the  glasses  prescribed  do  not 
give  sufficient  relief,  the  practitioner  naturally  asks  himself 
again  whether  the  fault  is  due  especially  to  the  intraoc- 
ular or  to  the  extraocular  muscles.  As  the  former  condition 
is  much  the  more  common,  he  naturally  reviews  his  data, 
asking  himself  whether  the  patient  has  an  actual  insufficient 
power  of  accommodation  (a  paresis),  or  whether  the  insuf- 
ficiency is  only  relative — that  is,  due  to  an  existing  hyperme- 
tropia  or  to  a  hypermetropic  astigmatism ;  or,  on  the  other 
hand,  whether  there  is  present  an  actual  excessive  power  of 
accommodation  (spasm),  or  a  relative  excessive  accommoda- 
tion such  as  exists  when  the  ciliary  muscle  is  in  a  practically 
normal  condition  in  a  myopic  eye. 

In  order  to  answer  these  questions,  which  relate  only  to 
the  condition  of  the  intraocular  muscles,  it  becomes  neces- 
sary, if  the  symptoms  warrant  it,  to  apply  a  minimum  dose 
of  atropin  sulphate,  and  by  measuring  its  effect  every  five 
or  ten  minutes  to  obtain  at  least  some  idea  of  the  condition 
of  the  ciliary  muscle.  Or  if  the  patient  returns  again,  and 
if  it  is  still  suspected  that  the  symptoms  are  due  to  some 
anomaly  of  the  accommodation,  it  may  then  appear  de- 
sirable to  measure  quite  exactly  the  range  of  relative  accom- 
modation. Until  all  these  measurements  have  been  made, 
and  still  others  here  described,  no  surgeon  can  honestly  con- 


384  Conclusions 

elude  that  in  a  given  case  he  has  collected  all  of  the  data 
which  relate  simply  to  the  intraocular  muscles. 

But  another  factor  in  our  problem  is  the  condition  of  the 
extraocular  muscles.  In  the  large  majority  of  cases,  of  course, 
no  instruments  or  other  apparatus  are  necessary  for  these  ex- 
cept the  prisms  of  the  trial  case,  or  such  modifications  of  them 
as  are  in  the  hands  of  most  practitioners.  But  in  certain  cases 
such  prisms  are  insufficient,  especially  for  the  measurement 
of  relative  convergence.  And  even  after  determining  that 
and  other  facts  of  the  same  class,  the  examination  of  the 
extraocular  muscles  is  not  completed  until  we  know  the 
amount  of  torsion  exercised,  whether  this  is  in  the  hori- 
zontal plane  or  in  some  other  which  is  inclined  to  the 
horizontal,  and  what  this  amount  of  torsion  is  with  varying 
degrees  of  convergence. 

But  after  every  possible  effort  has  been  made  to  deter- 
mine the  refraction  of  the  eye,  the  condition  of  the  intra- 
ocular muscles  and  of  the  extraocular  muscles — in  other 
words,  after  we  have  learned  all  we  can  concerning  the  eyes 
themselves,  and  after  the  most  careful  and  intelligent  efforts 
have  been  made  to  remove  the  cause  of  the  difficulty,  the 
patient  may  still  return  with  some  or  all  of  his  original 
asthenopic  symptoms.  Even  then  the  honest  practitioner 
must  admit  to  himself  that  he  has  not  learned  all  that  is  pos- 
sible in  regard  to  the  case.  He  remembers  that  the  principal 
factors  which  enter  into  the  act  of  comfortable  binocular 
vision  may  be  influenced  by  what  is  called  in  general  "  im- 
paired nutrition."  Accordingly  he  makes  tests,  or  has  them 
made,  to  determine  the  condition  of  the  stomach,  of  the 
kidneys,  of  the  blood,  or  otherwise  to  assure  himself  that  the 
functions  of  the  body  are  as  near  to  normal  as  possible. 

All  of  the  foregoing  tests  may  be  advisable  or  necessary 
simply  in  a  case  of  what  we  call  "  asthenopia."  But  again 
let  us  suppose  that  we  have  before  us  a  case  of  what  we  call 
in  general  terms  a  paralysis  of  one  or  more  of  the  extraocular 
muscles.  At  the  first  visit,  the  simpler  test  of  double  vision 
must  ordinarily  suffice.  If,  however,  we  wish  data  which  are 
reliable  for  an  exact  diagnosis,  it  is  desirable  to  measure  the 
field  of  fixation  with  the  perimeter  or  otherwise  much  more 


Conclusions  385 

exactly  than  is  ordinarily  done,  in  exceptional  cases  to 
measure  possibly  the  rapidity  of  the  lateral  movements,  or 
employ  other  means  to  assist  in  deciding  what  parts  of  the 
brain,  if  any,  are  involved. 

Finally,  let  us  suppose  that  we  have  before  us  still  a  third 
case,  such  as  is  usually  called  one  of  convergent  strabismus. 
Whatever  other  questions  may  arise  as  to  the  causation  or 
pathology  in  that  individual  case,  if  any  form  of  operative 
treatment  is  to  be  considered,  the  first  question  to  be  asked 
is,  as  we  shall  see  later,  Does  the  eye  turn  in  because  of  ex- 
cessive action  of  one  or  both  interni,  or  because  of  relaxation 
of  one  or  both  externi?  On  the  answer  to  this  question 
depends  the  decision  whether  we  are  to  make  some  form  of 
tenotomy  of  one  muscle,  or  advancement  of  another  muscle. 
In  order  to  answer  that  question  it  is  necessary,  as  has  been 
so  often  urged,  to  employ  methods  of  examination  which 
are  too  often  neglected,  but  which  have  been  described  in 
detail  in  the  foregoing  pages.  But  if  operation  is  decided 
upon,  we'  find  that  the  anatomical  studies  which  we  have 
made  give  us  valuable  suggestions  as  to  method  and  tech- 
nique. They  show  us  how  important  it  is  to  distinguish 
between  the  primary  and  secondary  insertions.  We  see  the 
importance  of  the  division  of  the  secondary  insertions  alone, 
or  of  the  primary  insertions  alone,  or  of  both  of  these  to- 
gether; or,  on  the  other  hand,  of  the  tucking  operation, 
or  of  advancement  in  its  different  forms. 

In  our  studies  thus  far  we  have  halted  frequently  to  take 
bearings  and  note  the  relation  of  the  subject  under  consider- 
ation to  the  demands  of  clinical  work.  Now  again,  after  re- 
viewing the  whole  theme,  we  have  observed  its  relation  to 
the  examination  of  thrte  of  the  most  common  types  of  cases 
with  which  we  have  to  deal.  It  is  hoped  that  these  examples 
indicate  at  least  in  some  degree  how  it  is  possible  to  make  our 
measurements  of  the  ocular  movements  much  more  thorough 
and  complete  than  is  ordinarily  done.  If  that  has  been 
made  clear,  we  have  prepared  a  foundation  of  anatomical 
and  physiological  data  for  our  clinical  work,  and  the  main 
object  of  this  first  part  of  our  study  has  been  accomplished. 


APPENDIX  A. 

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388  Muscles  of  the  Eye 

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SYMPATHETIC  NERVE. 

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(238)  1902.     Levinsohn.     "Uebei     den     Einfluss    des     Halssym- 
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(748)  1903.     Heine.      "Ueber  stereoskopische   Messung,"    Graefe's 
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(749)  1903.     Presas.     "Variedades  de  la  vision  binocular  estereo- 
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(75°)  *9°3-  Verhoeff.  "A  simple  test  for  stereoscopic  vision," 
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PUPILLARY  REACTION  WITH  CONVERGENCE. 

(751)  1902.     Marina.     "  Ueber  die  Pupillarreaktion  bei  der  Konver- 
genz," Neurolog.  Centralbl.,  S.  980. 

(752)  JpQS-     Marina.     "Ueber  die  Kontraktion  des   Spincter  iridis 
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der  Bulbi,"  Deutsche  Zeitschr.  f.  Nervenheilk.,  XXIV.,  Heft  3  und  4. 

ACCOMMODATION  AND  CONVERGENCE.    (GENERAL) 

(753)  J863.    Schurman,  J.  B.    "Vergelijkend  Onderzoekder  Beweg- 
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(757)  1886.  Secondi,  G.  "  Osservazioni  sul  rapporto  tra  1'  accom- 
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426  Muscles  of  the  Eye 

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INDEX   OF   THE   AUTHORS  REFERRED  TO 
IN  THE  BIBLIOGRAPHY 


Abadie,  683 
Aitken,  528 
Allis,  Phelps,  63 
Arlt,  F.,  754 
Arnaud,  J.,  538 
Astengo,  Giuseppe,  261 
Aub,  697 

Aubert,  H.,  450,  484 
Aubert  u.  Angelucci,  362 

B 

Bach,  L.,  167, 174, 175,  185,  186, 

192,  206,  210,  345 
Baehr,  C.  F.  W.,  434,  633,  669 
Bahr,  K.,  60 

Bannister,  J.  M.,  579,  725 
Barabaschew,  P.,  100 
Bardeleben,  14 
Barjardi,  358,  359 
Barker,  Lewellys  F.,  193 
Baxter,  W.  E.,  393,  529,  721,  820 
Beard,  310 
v.  Bechterew,  W.,  153,  176,  209, 

217,  593 

Becker,  Otto,  94,  95 
Beer,  Theodore,  296,  297,  319 
Belitzhy,  240 
Bell,  Ch.,  667 
Bellows,  48 
Below,  D.,  436 
Bennett  and  Clemesha,  375 
Berger,  E.,  93 
Berlin,  E.,  435,  636 
Bernard,  Cl.,  271 
Bernhardt,  123 
Bernheimer,  Stefan, 130, 138, 154, 

155,  161,  177,180181,195,203 
Berry,  G.  A.,  150,  342 
Berry,  P.,  788,  789 
Berthold,  E.,  444 
Besio,  Ed.,  323 
Bettremieu,  409 
Beyer,  388 

Bielschowsky,  A.,  350,  571,  743 
Bielschowsky  und  Ludwig,  580 


Biervliet,  182 

Bischoff,  E.,  432 

Bisinger,  H.,  778 

Bissell,  E.  J.,  408 

Bjerrum,  J.,  295 

Black.  Nelson  A.,  510 

Blanc,  L.,  61 

Bochdalek,  55,  70 

Boedicker,  J.,  207 

Bonnet,  A.,  4,  27,  28,  33 

Bottcher,  488 

Bottiger,  143 

Boucheron,  40 

Bouchut,  282 

Bourgeois,  58 

Bowditch,  H.  P.,  426 

Brailey,  505 

Brandes,  374 

Brandis,  P.,  162 

Braunstein,  343 

Bravais,  V.,  471 

Brawley,  265 

Bregmann,  213 

Brissaud,  E.,  156 

Brown,  A.,  614 

Brown,  E.  J.,  530 

Browning,  W.,  420 

Bruce,  131 

Budge,  34,  69 

Bull,  G.  J.,  360,  363,  744,  805 

Bumke,  351 

Bumstead,  511 

Burnett,  S.  M.,  706,  707,  711,  712, 

723 
Burow,  A.,  617,  690 


Charpentier,  A.,  646,  647 

Chibret,  364 

Chisolm,  J.  J.,  392 

Cohn,  H.,  73  (a) 

Coleman,  W.  F.,  394 

Collins,  365 

Conkey,  406 

Corning,  H.  K.,  246 

Cruveilhier,  37 

Crzellitzer,  A.,  305,  306,  307 


430 


Index  of  the  Authors 


Cuilloz,  354 
Curnow,  56 
v.  Cycon,  E.,  604 
Czermak,  562,  766 

D 

Dalrymple,  20 

Darkschewitsch,  117,  132 

Dastich,  J.,  631 

Davis,  A.  Edward,  504 

Delmas,  A.,  596 

Derby,  H.,  745 

Dobrowolsky,  W.,  352,  366,  389, 

473.  475 

Dodge,  Raymond,  468,  469 
Donders,  F.  C.,  260,  378,  416,  445, 

491,  492, 590,601,  621,  624,  634, 

639,  678,  681,  764, 765, 773. 775. 

776 

Donders,  F.  C.,  und  Dojer,  D.,  251 
Duane,  Alexander,  430,  539,  555, 

563,  566,  666,  694,  708,731 
Dupont,  346 
Duval,  M.,  107, 124,  212,  225,  335, 

609 
Dwight,  Thomas,  20,  47 


Eaton,  B.,  377,  461 
Eaton,  F.  B  .  558,  727 
Eaton  and  Halen,  552 
Edinger,  L.,  116,  118,  119,  144, 

187,   197 
Eichbaum,  45 
Einthoven,  324 
Elschnig,  A.,  87 
Erofew.  80 
Eversbusch,  83 
Exner,  S.,  331 
Eysselstein,  793 


Fano,  814 
Feddersen,  380 
Fereol,  606 
Fergus,  431 
Ferrall,  J.  M.,  31,  65 
Ferri,  594 
Ferrier,  818 
F£vrier,  L.,  595 
Fick,  A.,  625 
Fick,  E.  A.,  367,  440 
Fischer,  Karl,  795 
Flechsig,  P.,  336 
Fowler,  396 


Friedenberg,  P.,  99,  102 
Frohlich,  R.,  499 
Frost,  W.  A.,  402,  421 
Fuchs,  E.,  15,  84,  145,  283 
Fukala,  V.,  88,  311 


Galezowski,  403 

Gallopian,  Cl.,  493 

Gaudenzi,  C.,  459,  501,  540,  546 

Gaudenzi,  O.,  659 

van  Gehuchten,  A.,  148, 152,"! 66, 

184,  205,  215,  232 
Gerwer,  A.  W.,  502 
Giraud-Teulon,  599,  643,  682 
Golgi,  20 1 
Gould,  G.  M.,  531 
Gowers,  219 
Gradle,  H.,  512,  740 
v.  Graefe.  Albrecht,  269,  419,  448, 

7°9.  758,  767.  798 
Gray,  43 
Groenouw,  288 
Grossman,  Karl,  325 
Grunert,  89 
Griinhagen,  75,  76,  77 
Grut,  Edmund  Hansen,  507 
Guaita,  L.,  e  Bardelli,  L.,  407 
v.  Gudden,  112,  133,  198 

H 

Hallwachs,  746 

Hammersten,  Olof,  823 

Hampeln,  Paul,  78 

Hansell,  H.  F.,  395 

Harling,  26 

Harman,  N.  Bishop,  196,  236 

Harnack,  384 

v.  Hasner,  J.,  670,  671,  672 

Heese,  237,  341 

Hegg,  355 

Heine,  L.,  312,  313,  320,  747,  748 

Held,  146 

v.  Helmholtz,  H.,  257,  286,  443, 

446,487,  584,629,630,632,686, 

696,  772 

Henke,   W.,  270 
Henle,  7,  10,  105 
Hensen,  V.,  und  Volckers,  G.,  289 
Herbert,  J.  F.,  533,  534 
Hering,  Ewald,  447,  480,  490,  585, 

586,  640,  695,  771,  774,  779.  8n 
Herman,  280 
Hermann,  L.,  485,  691 
Herick.,  C.  L.,  168 
Herz,  M,  653 


432 


Index  of  the  Authors 


Hess,  C.,  103,  308,314,315,327, 
329,  483,  532,  800,  802,  807 

Hirschberg,  716 

Hitzig,  E.,  330,  600 

Hobby,  C.  M.,  724 

Hock,  J.,  489 

Hoffmann,  12,  259 

Holden,  Ward  A.,  422 

Hollstein,  8 

Holmes,  E.  L.,  273 

Hornemann,  Max,  457,  496 

Horner,  W.  E.,  64 

Hotz,  F.  C.,  398,  541,  703 

Howard,  A.,  547 

Howe,  Lucien,  21,  22,  72,  73,  344, 
400,478,482,553,  727,  734,  737, 
806,  808 

Hubbell,  548 

Huey,  474 

Huizinga,  J.  G.,  809 

Hulen,  Ward  H.,  732 

Hyrtl,  41 


Ivanhoff,  A.,  and  Arnold  J.,  81 

J 

Jackson,    E.,   410,  517,  518,  519, 

T  523.  535.  536,  735 
Jaesche,  E.,  575 
Javal,  E.,  356,  357,  520,  688 
Juler,  H.,  86 


K 


Kahler,  227 

Kahler  and  Pick,  106,  109,  no 

Kaplan,  L.,  und  Finkelnburg,  222 

Katz,  613 

Kausch,  157 

Kaush,  204 

Kerschbaumer,  Rosa,  85 

Kiribuchi,  90 

Knapp,  H.,  401 

Knies,  M.,  139    649 

Knoll,  Ph.,  338 

v.  Koelliker,  A.,  113,  137,  149, 151 

202,  214,  223,  228 
Koller,  372,  399 
Kostareff,  824 
Koster,  80 1 
Koyle,  Frank  H.,  572 
Krause,  C.  F.,  618 
Kugel,  L.,  668 
Kunn,  C.,  411 
Kuyper,  379 


Laborde,  607,  610 

Laborde  et  Duval,  6n 

Lamansky,  S.,  467 

Lamare,  472 

Landesberg,  371 

Landolt,  E.,    328,    418,  424,425, 

455. 592. 644, 654, 655, 676, 717, 

783,  813 

Landouzy,  L.,  233 
Lange,  O.,  298 
Langer,  Fr.,  53 
Langly  und  Anderson,  326 
Laura,  220 
Lawford,  U.  B.,  73 
Le  Conte,  Joseph,  272,494,641, 

812,  815 

Le  Double,  59,  476 
Leube,  121 
Levinsohn,  George,  238241/347, 

385.  479 

Lewin  und  Guillery,  386 
Liebreich,  n 
Linhart,  35 
Lockwood,  C.  B.,  46 
Lodato,  242 
Lohnstein,  316 
Ludwig,  Carl,  821 


M 


Macalister   57 

MacGillayry,  768 

Mackenzie,  32 

Maddox,  Ernest  E.,  2 63 ,'284, "382, 

508,  521,  544,  785,  819 
Magnani  e  Lavagna,  728 
Mannhardt,  J.,  755 
Marchi,  114 

Marina,  188,  348,  751,^752 
Marinesco,  231 
Marlow,  576 
Marshall,  Milnes,  247 
Martin,  373,  506 
Maulder,  591,  700 
Mauthner,  L.,  199,  337,!?339.  756 
Mayer,  C.,  229 
Mazza,  140 
Medida,  470 
Meissner,  George,  441,  442,  582, 

626,  627,  680,  685 
Mendel,  226 
Mercier,  Ch.,  274 
Merkel,  Fr.,  3,  38,  79,    211 
Merkel  and  Kallius,  23 
Meyerhof,  477 
Michel,  A.,  82,  292 


Index  of  the  Authors 


433 


Mingazzini,  221 

van  Moll,  684 

Monakow,  Dr.  C.  v.,  178 

Monoyer,  299 

Morabito,  513 

Moseley,  N.  B.,  67 

Motais,  16,  17,  18,  50,  463 

Mott,  650 

Muller,  H.,  68 

Miiller,    Johan,    252,  267,   699, 

760 

Munk,  H.,  651 
Munz,  Martin,  2 


X 


Nagel,  Albrecht,s87,  588,  704,  780, 

781,  810 

Nicati,  W.,  451,  453 
Noyes,  Henry  D.,  705 
Nussbaum,  J.,  340 
i,  M., 


Nussbaum, 


122,  249, 


Obersteiner,  108,  147,  208,  216,  224 

Obregia,  652 

Oppenheim,  134 

Orchanski,  J.,  427 

Orlandini,  733 

Ostwalt,  522,  542 

Ovio,  675 

Owen,  Richard,  244 


Pacetti,  158 

Panas,  464 

Panegrossi,  183 

Panni,  564 

Panum,  769 

Parinaud,  120 

Payne,  524 

Percival,  Archibald,  710,  713,  792 

Pereles,  787 

Perlia,  135 

Pfalz,  730 

Pfluger,  368,  381 

Pickert,  M.,  578 

Pilz,  J-,  9 

Pineles,  169 

Plateau,  761 

Plotke,  Ludwig,  281 

Pohlmann,  376 

Porterfield,  770 

Pravaz,  616 

Prentice,  Ch.  F.,  714 

Presas,  749 


Prince,  A.  E.,  525 
Probst,  189 
Purkinje,  268 
Pyle,  736 


Quain,  42 


Q 


R 


Raehlmann  und  Witkowsky,  275, 

„  333,  777 

Ramon  y  Cajal,  S.,  06 

Randall,  B.  Alex.,  715 

Raudnitz,  412 

Raymond,  Emil  Du  Bois,  822 

Reboud,  285 

v.  Recklinghausen,  F.,  486 

Reddingius,  287,  803 

Remy,  559,  560,  561,  565 

Reute,  Th.,  414,  415,  622 

Reymond,  369,  370,  574,  784 

Reymond  et  Stilling,  786 

Risley,  Samuel  D.,  383,387,391, 

509.  537,  557 
Ritzmann,  E.,  603,  605 
Robinski,  S.,  97 
Roosa,  D.  B.  St.  John,  465 
Rouviere,  H.,  49 
Rudin,  W.,  404 
Ruete,  6,    762 
Russell,  J.   S.  R.,  159,  497,  656, 

657,  658 
Ryder,  John  A.,  248 


Sabin,  Florence  R.,  194 
Sachs,  Th.,  163,  170,  794 
Salomonsohn,  235 
Samelsohn,  J.,  602 
Sander,  W.,  277,  334, 
Sappey,  C.,  39,  52 
Sargent,  Dudley  A.,  824 
Savage,  G.  C.,  462,  495,  556,  612, 

661,  796,  816,  817 
Schiefferdecker;  24 
Schild,  567 
Schiotz,  H.,  729 
Schiotz,  M.  O.,  549,  731 
Schirmer,  349 
Schjachtin,  689 
Schmiedt,  W.,  797 
Schneller,   19,   190,258,438   445. 

452,  454,  664 
Schon,  W.,  417,  701,  702 
Schultz,  413 
Schurman,  J.  B.,  753 


434 


Index  of  the  Authors 


Schwalbe,  G.,  13,  52,  98,  in 

Schwalbe,  H.,  171,  179 

de   Schweinitz,  243 

Seashore,  C.  E.,  290 

Secondi,  G.,  648,  757,  790 

Shaw  and  Thompson,  172 

Sherrington,  C.  S.,  673 

Shimanura,  S.,  164 

Siemens,  F.,  278 

Siemerling,  141,  173 

Silex,  P.,  503 

Simon,  742 

Skrebitzky,  A.,  635,  637 

Since,  Alfred,  481 

Smith,  Priestley,  262,  500 

Snellen,  H.,  Jr.,  390,  791 

Sommering,  Sam.  Thorn,  v.,  5 

Sous,  G.,  739 

Spitzka,  E.  C.,  125,  136 

Stadfeldt,  A.,  309 

Stark,  C.,  332 

Starr,  M.  Allen,  126,  127 

Staurenghi,  200 

Steulp,  O.,  165 

Stevens,  G.  T.,  458,460,514,515, 

_  799.543.545.573.598,718,726 

Stevenson,  568 

Stilling,  115 

Stock,  91 

Strangeways,  J.,  70  (a) 

Straub,  M.,  397,  516,  719,  741 

Struthers,  J,  66 


Taylor,  J.  B.,  234 

Tenon,  27 

Terrien  et  Camus,  239 

Testut,  L.,  71 

Theobald,  526,  569 

Thomas,  W.  Ernest,  62 

Tourtual,  615,  623 

Treutler,  318 

Tscherning,  M.,  101,  264,  291,  293, 

300, 301, 302,  303, 321,  322,  645, 

692,    693 

Tupper,  J.  L.,  254 
Turner,  Dr.  Aldren.  230 


Ulrich,  L.,  782 

V 

Valentin,  619 
Valk,  Fr.,  437 
Vamossy,  Z.,  405 
Van  der  Brugh,  804 
Vard,  H.  Hulen,  734 
Verhoeff,  550,  551,  554,  750 
Vialet,  M.,  1 60 
Virchow,  Hans,  51 
Volkers  u.  Hensen,  104 
Volkmann,  A.  W.,  433,  466,  581, 
620,  677,  679,  687,  763 

W 

Wallace,  J.,  74,  527,  720 

Wallenberg,  218 

Warner,  F.,  276,  608 

Weber,  E.  H.,  759 

Weber,  M.,  245 

Weiland,  Carl,  439,  498,  662 

Weinhold,  665 

Weiss,  G.,  255,  304 

Wells,  Soelberg,  36,  44 

Westien,  H.,  423 

Westphal,  C.,  128,  129,  142 

Wiers,  577 

Wilbrand  u.  Saenger,  191 

Williams,  Charles  H.,  266,  570 

Wilmart,  L.,  663 

Wilmart,  S.,  674 

Wintersteiner,  294 

Witkowski,  L.,  279,  642 

Woinow,  M.,  253,  353,  361,  589, 

638,  698 

Wolff,  Hugo,  25 
Wood,  C.  A.,  456 
Wundt,  W.,  583,  628 


v.  Zehender,  W.,  256,  738 
Ziegler,  S.  L.,  722 
Zietzschmann,  250 
Zoth,  660 


APPENDIX  B. 
SUBJECTS  FOR  STUDY. 

Reasons  for  this  List  of  Questions. — As  a  traveler  finds 
numerous  paths  leading  from  the  main  road  which  he  is 
pursuing,  so  in  this  study  questions  connected  with  the  main 
topic  have  often  arisen.  At  first  they  were  noted  with  the 
expectation  of  returning  to  them.  But  it  soon  became 
evident  that  one  life  was  far  too  short  for  working  out  all  the 
unsolved  problems  relating  to  the  ocular  muscles.  A  few  of 
these  questions  are  therefore  added  in  the  hope  that  they  may 
prove  suggestive  to  future  students.  For  it  is  unfortunately 
evident  that  much  energy  and  professional  zeal  run  to  waste 
for  lack  of  intelligent  guidance.  Our  ranks  are  recruited 
each  year  by  ambitious  men,  well  equipped  for  their  work, 
and  with  ample  time  for  investigation,  especially  in  their  early 
years  of  practice.  As  the  entire  field  is  fresh  to  them,  they 
turn  to  whatever  part  is  of  special  interest,  too  often  making 
the  great  mistake  of  not  ascertaining  first  what  has  been 
accomplished  by  other  workers.  The  result  is  a  loss  to  our 
science  of  valuable  energy  and  patient  labor  and  a  disappoint- 
ment to  the  student,  when  he  is  shown  later  that  his  "new 
truths"  were  discovered  years  before.  As  the  same  mistake 
is  often  made  also  by  those  of  us  who  are  old  enough  to  know 
better,  we  find  our  literature  full  of  repetitions.  This  is 
particularly  so  in  America,  and  pre-eminently,  it  would 
seem,  in  articles  relating  to  phases  of  so-called  eye-strain. 
Therefore  it  may  be  a  convenience  for  those  who  possess  the 
desire  and  opportunity  to  study  this  subject  further,  to  have 
suggestions  as  to  at  least  a  very  few  of  the  problems  which 
yet  remain  to  be  solved.  As  in  certain  branches  of  manufac- 
ture the  by-products  become  ultimately  more  important  than 
the  substance  which  first  was  made,  so  in  this  list  of  subjects 
for  study  it  is  hoped  that  the  results  obtained  may  prove 
much  more  valuable  than  the  work  which  has  called  attention 
to  them. 

435 


436  Appendix  B 

QUESTIONS. 

Can  Kaiserling's  method  for  preserving  specimens  be  improved — 
especially  in  preventing  the  specimen  from  becoming  hard  ? 

What  stain  of  connective  tissue  can  be  found  which  is  more  selective 
than  those  we  now  have,  and  in  colors  which  show  better  in  photo- 
graphs? 

Measure  a  considerable  number  of  globes  according  to  the  method 
here  outlined  to  ascertain  the  average  position,  length,  and  curve  of  the 
primary  insertions  of  the  recti. 

In  three  or  four  orbits  inflate  the  globe,  make  moderate  traction  of 
the  internal  rectus,  or  extreme  traction,  harden  the  specimens  with 
the  globes  in  these  different  degrees  of  adduction,  make  horizontal 
sections,  and  observe  the  exact  condition  of  the  check  ligaments. 

Make  a  series  of  horizontal  sections  showing  the  details  of  Horner's 
muscle. 

Show  the  action  of  Horner's  muscle  and  its  exact  effect  on  the 
sinking  of  the  caruncle  after  tenotomy  of  the  internal  rectus. 

Of  what  practical  importance,  if  any,  is  the  sound  produced  by 
the  eye  muscles? 

What  is  the  exact  location  of  cells  in  the  cortex  of  the  brain  which 
give  rise  to  nerve  fibers  supplying  the  muscles? 

Do  the  experiments  of  Ferrier  show  conclusively  the  existence  of 
motor  centers  in  the  cortex? 

What  bands  of  fibers  are  there  which  pass  from  the  cortex  or  other 
portions  of  the  brain  to  the  nucleus  of  the  motor  oculi? 

What  further  evidence  can  we  obtain  by  the  degeneration  experi- 
ments of  Von  Gudden  to  ascertain  which  cells  in  the  nucleus  of  the 
motor  oculi  preside  over  certain  muscles? 

Demonstrate  the  anastomoses  between  the  third  and  other  nerves. 

When  operating  on  different  members  of  the  same  family  who  have 
squinting  eyes,  measure  accurately  the  position,  length,  and  form  of 
the  arc  of  the  primary  insertion  of  the  muscle  which  is  divided. 

How  many  conjugate  innervations  are  there  and  what  proof  of  each? 

With  the  aid  of  the  ophthalmophacometer,  measure  the  size  of  the 
angle  alpha  and  the  tipping  of  the  lens  in  a  considerable  number.  A. 
Of  normal  eyes.  B.  Of  eyes  in  asthenopic  persons. 

What  is  the  relation  of  astigmatism  to  the  size  of  the  angle  alpha? 

What  are  the  points  of  origin  and  insertion  of  the  fibers  constituting 
the  Zone  of  Zinn  as  they  pass  from  certain  parts  of  the  ciliary  process 
to  the  anterior  and  posterior  portions  of  the  lens? 

Does  the  Zone  of  Zinn  vary  greatly  in  different  individuals? 

Repeat  the  observations  of  Crzellitzer  and  Stadfelt  to  see 
whether  the  anterior  surface  of  the  lens  becomes  more  convex  during 
accommodation. 

Repeat  the  experiments  of  Volcker  and  Hansen  to  ascertain  the 
changes:  A.  In  the  choroid.  B.  In  the  posterior  surface  of  the  lens, 
if  any  occur  during  the  act  of  accommodation. 


Appendix  B  437 

With  the  aid  of  Tscherning's  ophthalmophacometer,  observe  the 
changes  in  the  entoptic  images  during  the  act  of  accommodation  and 
give  an  explanation  of  them. 

Repeat  the  experiments  of  Heine  to  verify  his  statement  concerning 
the  falling  of  the  lens  during  the  act  of  accommodation. 

With  the  aid  of  the  ophthalmophacometer  observe  the  apparent 
astigmatic  accommodation  which  occurs  in:  A.  Normal  eyes.  B.  In 
eyes  of  asthenopic  persons. 

Measure  the  torm  and  the  degree  of  malposition  of  the  lens  in 
different  members  of  the  same  family  who  suffer  from  obstinate 
forms  of  eye  strain. 

Observe  more  accurately  the  characteristic  contraction  of  the  pupil 
in  different  individuals. 

Repeat  the  experiments  of  applying  atropin  and  eserin  to  the  eyes 
of.  animals,  removing  the  eyes,  freezing  them  immediately,  and  making 
sections  to  determine  the  form  of  the  lens  and  condition  of  the  ciliary 
muscles. 

What  difference  is  there  in  the  action  of  a  given  amount  of  any 
cycloplegic  or  myotic  upon  the  ciliary  processes  of  individuals  of 
different  ages? 

What  curve  do  we  obtain  for  the  relaxation  of  the  accommodation 
and  dilation  of  the  pupil  after  the  use  of  very  weak  solutions  of 
duboisia,  scopolamin,  and  of  similar  drugs? 

Exactly  what  connection  is  there  physiologically  between  the  pos- 
terior fibers  of  the  occipito-frontalis  and  the  trapezius  which  may 
account  for  the  pain  which  extends  from  the  occiput  over  the 
shoulders  when  prolonged  efforts  at  accommodation  are  made? 

In  any  considerable  number  of  cases  what  is  the  lifting  power  of  the 
adductors? 

What  better  method  can  be  proposed  for  measuring  the  lifting 
power  of  the  adductors? 

What  is  the  usual  lifting  power  of  the  adductors  in  youth?  In 
middle  life?  In  old  age? 

Does  the  lifting  power  of  adductors  vary  in  proportion  to  the  mus- 
cular development  of  the  individual? 

What  is  the  amount  of  muscular  force  expressed  in  grams  which  is 
necessary  to  rotate  an  eye  outward:  A.  In  esophoria  of  a  certain 
degree?  B.  In  exophoria  of  a  certain  degree ? 

Is  the  relation  of  torsion  to  accommodation  the  same  as  the  relation 
of  torsion  to  convergence? 

What  better  explanation  can  we  give  of  the  mechanism  of  associated 
lateral  movements  than  the  one  which  we  now  have  ? 

Of  what  clinical  importance  is  the  difference  between  the  binocular 
and  monocular  near  point  ? 

What  simpler  methods  can  be  found  for  the  measurement  of  relative 
accommodation  ? 

In  a  considerable  number  of  emmetropes  what  line  or  curve 
represents  their  average  range  of  relative  convergence  ? 


438  Appendix  B 

What  are  the  causes  of  the  apparent  variation  of  relative  convergence 
among  different  emmetropes  ? 

When  measurements  are  made  of  a  considerable  number  of  em- 
metropic  eyes  as  to  the  degree  of  torsion  with  convergence  in  the 
horizontal  plane  and  in  oblique  planes,  do  the  figures  thus  obtained 
accord  with  those  given  by  Landolt? 

What  better  methods  can  be  found  for  the  measurement  of  relative 
torsion  ? 

In  a  considerable  number  of  emmetropes  exactly  what  relation 
exists  between  the  strength  of  the  recti,  as  expressed  in  prisms,  and 
the  strength  of  the  indi/idual  as  expressed  in  foot  pounds? 


APPENDIX  C. 

OPHTHALMOLOGICAL  JOURNALS  IN  CERTAIN  AMERICAN 
LIBRARIES. 

The  student  who  cares  to  pursue  this  subject  further,  may 
perhaps  see  articles  in  the  bibliography  which  he  thinks  would 
particularly  interest  him.  It  is  true  that  volumes  can  often  be 
obtained  from  the  Library  of  the  Surgeon  General  at  Washington, 
either  directly  or  through  local  incorporated  libraries,  and  the 
opportunity  thus  afforded  to  the  humblest  student  is  a  source  of 
congratulation  to  the  medical  profession  of  the  country.  There 
are,  however,  numerous  inconveniences  in  obtaining  books  from 
this  source.  It  involves,  of  course,  delay  and  expense  of  trans- 
portation, and  the  volumes  can  be  kept  but  a  short  time,  as  the 
purpose  of  that  library  is  avowedly  not  to  send  books  out,  but  to 
keep  them  for  consultation  in  Washington. 

It  has  therefore  seemed  desirable  to  ascertain  where  files  of  the 
ophthalmological  journals  are  to  be  found,  and  although  this  list  of 
periodicals  is  not  quite  complete,  it  is  probably  sufficient  for  the 
needs  of  most  students.  The  numbers  in  the  first  column  on  each 
page  indicate  the  name  of  the  periodical  as  that  is  given  in  the 
list.  As  the  last  column  on  the  page  following  this  one  shows 
the  year  when  each  periodical  first  appeared,  it  is  possible,  by 
counting  from  that  date,  to  determine  the  number  of  any 
volume;  or  if  the  number  of  the  volume  is  given,  the  year  can  be 
determined.  This  list  may  prove  convenient  not  only  to  the 
student  of  the  muscles  of  the  eye,  but  for  other  departments  of 
ophthalmology. 


439 


440 


Appendix  C 


LIST  OF  PERIODICALS. 


First  Vol. 
Published 


1.  American  Journal  of  Ophthalmology 

2.  Anales  de  Oftalmologia  (Mexico) 

3.  Annales  d'Oculistique 

4.  Annali  di  Ottalmologia 

5.  Annals  of  Ophthalmology . , 

6.  Archives  d'Ophtalmologie. * 

7.  Archives  of  Ophthalmology 

8.  Archivio  di  Ottal.  (Palermo) 

9.  Beitrage  zur  Augenheilkunde 

10.  Bericht  der  Ophthal.  Gesellschaft,  Heidelberg 

11.  Bolletino  d'Oculistica  (Firenze) 

12.  Bulletin  Societe  Frai^aise  d'Ophtalmologie 

13.  Centralblatt  ftlr  praktische  Augenheilkunde 

14.  Clinica  Oculistica 

15.  Clinique  Ophtalmologique 

16.  Graefe's  Archiv  fur  Ophthalmologie 

17.  Klinische  Monatsblatter  fur  Augenheilkunde 

1 8.  Nagel's  Jahresbericht  der  Ophthalmologie 

19.  Nederl.  Tijdsch.  v.  Geneeskunde 

20.  Ophthalmic  Record 

21.  Ophthalmic  Review 

22.  Ophthalmic  Year  Book 

23.  Ophthalmologische  Klinik 

24.  Ophthalmology 

25.  Ophthalmoscope,  The , 

26.  Proceedings  Western  Ophthal.  and  Otolog.  Association. 

27.  Recueil  d'Ophtalmologie 

28.  Reports  Royal  Lond.  Ophthal.  Hospital 

29.  Revue  General  d'Ophtalmologie 

30.  Trans.  Internat.  Ophthal.  Congress 

31.  Trans,  of  the  American  Ophthalmological  Society 

32.  Trans.  Ophthal.  Society  of  the  United  Kingdom 

33.  Trans.  Section  Ophthal.  American  Medical  Assoc 

34.  Wochenschr.  f.  Ther.  u.  Hyg.  des  Auges 

35.  Zeitschrift  filr  Augenheilkunde 


1884 
1900 
1838* 
1871 
1892 
1880 
1869 
1893 
1895 
1877 
1870 
1883 
1897 
1900 
1895 
i  1854 
i  1863 

1857 

1881 
1904 
1897 
1904 
1894 
1897 
1873 
1857 
1882 
1857 
1865 
1880 
1891 
1897 
1899 


*  2  vols.  a  year. 


Appendix  C 


441 


ANN  ARBOR,  MICH. 
STATK  UNIVKRSITV. 

BALTIMORE,  MD. 
JOHNS  HOPKINS  LIBRARY. 

BOSTON,  MASS. 
BOSTON  MEDICAL  LIBRARY. 

Begin- 
ning. 

Ending. 

Com- 
plete. 

Begin- 
ning. 

Ending. 

Com- 
plete. 

Begin- 
ning. 

Ending. 

Com- 
plete. 

I 

2 

3 
4 
5 
6 

7 
8 

9 

10 

II 

12 
13 
14 
15 

16 

17 
18 
19 
20 

21 
22 
23 
24 
25 
26 
27 
28 
2Q 
30 
31 
32 

33 
34 
3 

V.   I 

To  date 

Yes 

Yes 
Yes 

Yes 
Yes 

Yes 
Yes 

Yes 
Yes 

No 
Yes 
Yes 

Yes 
Yes 

No 

Yes 

Yes 

Yes 

V.  I 

V.  20 

Yes 

No 

Yes 
No 
Yes 

Yes 
Yes 
No 

Yes 
Yes 

Yes 

Yes 

No 

Yes 
Yes 

V.  106 

V.  35 

V.  7 

V.  116 

V.35 
To  date 

V.47 

V.  134 

V.  6 
V.  i 

V.  8 

V.  14 

V.  25 

V.34 

V.  i 
V.  i 

To  date 
To  date 

V.  i 

V.4 

V.  i 

V.  29 

V.  3 
V.  i 

V.9 
V.39 
V.  i 

V.  ii 
V.  62 
V.  15 

To  date 

V.  I 

To  date 

V.  2 

To  date 

V.  i 
V.i 

V.  14 
V.  24 

V.  2 

V.  I 

V.i 

V.  2 

V.  2 

To  date 

V.  I 
V.  i 

V.  16 

To  date 

V.  3 

V.  2 

V.  27 
V.9 

V.  i 
V.  i 

V.7 

V.  8 
To  date 

V.  i 

To  date 

V.  i 

To  date 

V.9 
V.  I 

To  date 
To  date 

V.  i 

To  date 

442 


Appendix  C 


CHICAGO,  ILL. 
COLL.  OF  PHY.  AND  SURGEONS. 

CHICAGO,  ILL. 
RUSH  MED.  LIBRARY. 

CINCINNATI,  OHIO. 
PUBLIC  LIBRARY. 

I 

2 

,3 

4 

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6 

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18 
K) 

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35 

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Ending. 

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plete. 

Begin- 
ning. 

Ending. 

Com- 
plete. 

Begin- 
ning. 

Ending. 

Com- 
plete. 

V.3 

V.  8 

No 
No 
No 
No 

No 
No 

Yes 

No 

No 
Yes 

No 

No 

No 

No 

No 

No 

No 

Yes 
No 

V.  67 

V.  115 

V.  121 

V.  136 

V  5 

V.  14 

V.  19 

V.  27 

V.  23 

V.  28 

V.  I 

V.3 

V.5 

V.  23 

V.  24 

V.  30 

V.  40 

V.  48 

V.43 

V.37 

V.  64 

V.  44 

.      .... 

V.5 

V.  i 
V.9 
V.  i 

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V.  18 

V.  8 
V.  i 

V.  15 

V.  22 

V.  i 

V.  2 

V.  8 

V.  12 

V.  i 

To  date 

V.  i 

V.  i 

V.3 

Appendix  C 


443 


GALVESTON,  TEXAS. 
LIB.  OF  MED.  UNIV.  OF  TEXAS. 

NEW  YORK,  N.  Y. 
"I.Y.  ACADF.MY  OF  MEDICINE. 

PHILADELPHIA,  PA. 
COLLEGE  OF  PHYSICIANS. 

I 
2 
3 
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5 
6 

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Ending. 

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plete. 

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Ending. 

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plete. 

Begin- 
ning. 

Ending. 

Com- 
plete. 

No 
No 

Yes 

No 

No 

V.  I 

To  date 

Yes 

Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 

Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
Yes 
No 
Yes 
Yes 
Yes 
No 
Yes 
Yes 
Yes 
Yes 
Yes 

V.  I 
V.  I 
V.  I 
V.  I7 
V.  i 
V.  i 
V.  i 
V.  i 
V.  i 
V.  10 
V.  i 

V.  18 
V.  8 
V.  134 

Yes 
Yes 
Yes 

Yes 
Yes 
Yes 
No 
Yes 
Yes 
No 

Yes 

No 
Yes 
No 
Yes 
Yes 
Yes 
Yes 

Yes 
Yes 
Yes 

No 
Yes 

No 
No 
No 
Yes 
No 
Yes 
Yes 

V.  I 
V.  I 
V.  I 

V.  i 
V.  I 
V.  i 
V.  i 
V.  i 

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To  date 
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V.  II 

V.  12 

V.  14 
V.  25 

V.  34 

V.   12 

V.  7 
V.  32 

V.  8 

V.  30 

V.  i 
V.  i 

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V.  i 
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V.  i 
V.  i 
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V.  3 

V.  29 

V.  i 
V.  i 
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V.  63 
V.  44 
V.  35 
V.  39 
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V.  8 
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.... 

V.  3 

To  date 

444 


Appendix  C 


.ST.  LOUIS,  MO. 
ST.  Louis  Mho.  LIBRARY  ASSN 

WASHINGTON,  B.C. 
LIB  ,  SURG.-GKN.'S  OFFICE. 

WORCESTER,  MASS. 
DISTRICT  MED.  SOCIKTY. 

Begin- 
ning. 

Ending. 

Com- 
plete. 

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ning. 

Ending. 

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plete. 

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ning. 

Ending. 

Lcm- 
plete. 

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Yes 
No 

No 

V.  I 

V.3 
V.I 
V.  i 
V.  i 
V.  i 
V.  10 
V.  i 
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V.  34 

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V.  5 
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V.  47 

Yes 

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No 
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i 

! 

INDEX  OF  AUTHORS. 


Adachi  24,  37. 
Albinus  52. 
Anel  3,   4. 
Aubert    258. 
Axenfeld  107,  114. 

B 

Bannister  241,  242,  301. 

Barker  85,  86. 

Beer  141. 

Bernard,  Claude  102. 

Bernheimer  89,90,91,92,94,  97, 

98,    99,    254. 

Bielschowski    152,    241,    242. 
Bisinger  290,  315,  325,  328. 
Bochdalek  52,  114. 
Bourchard   315. 
Bowman  58. 
Brevvster  306,  307. 
Budge  52. 
Bull  306. 
Burnett  282. 


Corning  116. 
Crete  283. 
Crzellitzer  139,  140. 

D 

Darwin  82,   159. 

Dennett  281. 

Derby  306,  307. 

De  Schweinitz  193. 

Dianoux  201,  202. 

Dobrowolsky   154. 

Dodge  203,  207,  209. 

Dojer  123. 

Donders  123,  128,  141,  144,  145, 
146,  159,  160,  161,  163,  164,  174, 
180, 183, 185,  259,  268,  273,  274 

279,  296,  32S.  328,  330.  336,  338, 

348,  351. 

Duane  180,   196,   259. 
Dwight,  it,  1 8. 


E 

Eaton  193. 
Edinger  88,  89,  106 
Eperon,  196. 


Feddersen  164. 
Ferrier  93. 
Flower  96,  104. 
Friedenberg   65. 
Fuchs  23,  28,  33,  37,  59. 

G 

Galezowski  219. 

Graefe  14,  24,117,224,225,238, 

3°4,   347- 
Gray  80,  8r,  82,  85,87,95,104, 

108. 

Grief swald  52. 
Grossman  139. 
Grunert  59. 

Grut,  Edmund  Hansen  14,  219. 
Gull  strand  77. 


H 


Halleck   166. 

Hansel  281. 

Heine  138. 

Helmholtz  67,  78,  79,122,136,137, 
141,  143,  150,  202,  244,  248,  258, 
268,  274,  275,  279,  296,  340. 

Henle  80,  81,  82,  84. 

Hensen  143. 

Herbert  226. 

Hering  78,  196,  212,  244,  253,  257, 
258,  259,  296,  348,  351. 

Hess  64,  65,  78,  138,  140,  145,  217, 

329.  33°.  331-  332,  335-  336- 
Horner  43,  51,  52,  113. 
Hotz  17. 
Howe  37,  183,184,216,218,223, 

241,  249,  284,  314,  330,  355,  356, 

354- 

Hubbell  239. 
Hunter  112. 


445 


446 


Index  of  Authors 


Iwanoff  56,  57,  58. 

J 

Jackson  282,  283. 
Javal  69,  70,  75,  124,  131,  134,  211, 
3J6,  376,  377- 

K 

Kaiserling  3,  4,  6,  n. 

Kallius  13,  15,  31,  39,  41,  45,  48. 

Kendall  166. 

Knapp   183,   185. 

Kuster  196. 

Kuyper  160,  165,  166. 


Landolt4i,  121,  127,  128,  129,  143, 
144,  145,  162,  181,  182,  183,  196, 
197,  219,  222,  258,  293,  296,  300, 
351,  360,  361,  362,  367,  369. 

Le  Conte  272,  352,  354,  359- 

Listing  120,  121,  122,  180,  182,  183, 
184,  185,  268,  273,  274. 

Lockwood  31,  32,  49. 

Ludwig  241. 


M 


Maddox4i,  128,  130, 134,  193,  211, 

221,  228,  229,  230,  231,  232,  238, 

259,  260,  261,  268,  272. 
Mallory  5. 
Marshall  115. 
Mauthner    105,    274. 
Meissner  267. 
Mendel  101,  116. 
Mercator  33,  34,  38. 
Merkel  12,  13,  15,  31,  35,  39,  41,  45, 

48,  50,  63,  84,  103. 
Mivart  in. 
Motais  5,  17,  41,  42,  44,  49,  50,  51, 

in. 
Mueller,  Johannes  17,  58,  214. 


N 


Nagel  292,  293,  296,  325,  329,  345. 

Nicati   193. 

Noland  62. 

Norris    293. 

Norris  and  Oliver  162. 

Noyes  283,  284,  299,  301,  306. 


Oliver  293. 


Panas  202. 

Pereles  329,  330,  333. 

Pereyni  6. 

Perlia  92. 

Petit  112. 

Prentice  281. 


Ramon  y  Cajal  94. 

Remy  235,  236. 

Retzius  61. 

Ribbert  5. 

Richard  306. 

Ridgeway  234. 

Risley  228,  283,  286,  299,  301. 

Ruete  265,  274. 

Russell  94. 

Ryder  114. 


Sabin  90. 

Sargent  373. 

Savage  224,  246. 

Schiefferdecker  14. 

Schiotz  124,  377. 

Schneller  20,  23,24,39,186,196. 

Schoen  61,  62. 

Schreiner  330. 

Schirmer  149,  152. 

Schuurmann  196,  197. 

Sohwalbe  17,  44. 

Smee  215. 

Snellen  163. 

Stadfeldt    139. 

Starr,  M.  Allen  99,  100. 

Stevens  54,  193, 194, 224,  228,229, 

233.  237.  238.  239.  247.  248,  250, 

260,  261,  283. 
Stokes  228. 
Straub  169,  173. 
Sylvius  88,  90,  93,  102,  109. 


Tenon  20,  23,  25,  26,  34,  35,  36,  40 

43.  44,  45-  47- 
Testut  82. 
Thorington  222. 


Index  of  Authors 


447 


Tscherning  66,  69,  72,  73,  75,  128, 

i3r»  *34.  137.  !38.  r39.  r4o,  142, 
143,   270,  271,  274.  302. 
Tyndall  117. 


U 


Uhthoff  131. 
Unna    5. 


Van  Giesen  5. 

Verhoeff  58,  262. 

Virchow,  Hans  35,  41,43,44,51. 

Voelckers  143. 

Volkmann  23,  27,  78,  196,  202,  244, 

246,  247,  248,  249,  261,  262,  263, 

264,  35i.  352,  379- 


Von  Gudden  91,  96,  101. 
W 

Wallace  58. 
Weiland  186. 
Weiss  37. 

Wheatstone  305,  306. 
Williams,  Chas.  H.  241,  374. 
Woinow  154. 
Wolff  17. 
Wood  189. 


Zeigler  282. 

Zeiss  64,  139. 

Zenker  5. 

Zinn  12,  17,  22,  58,  60,  61,  62. 


INDEX  OF  SUBJECTS. 


Abducens,  sixth  nerve,  105 
Abduction,  definition  of,  179 
Abductive  prism,  281 
Accessory  muscles  of  accommoda- 
tion, 80 
Accommodation,  136 

accessory  muscles  of,  80 

act  of,  137 

anterior  surface  of  lens  in,  142 

astigmatic,    153,  1 54 

change  during,  137 

efforts  at,  149 

illustration  of  ranges,  309 

in  lower  animals,  140 

posterior  surface  of  lens  in,  143 

range  of,  144 

relation  of  convergence  to,  341 

relation  to  torsion,  348 

relative,  309 

relative,  blank  for  measurement 

of  •  3 1 7 

relative,  desiderata  for  measure- 
ment of,  313 

relative,  how  to  plot,  325 
relative,  influenced  by  age,  333 
relative,  measurement  for  clini- 
cal purposes,  336 
relative,  measurementwithopto- 

meter,  317 

relative,  other  methods  of  meas- 
uring, 328 

relative,  Pereles'  method,  329 
relative,  table,  319 
resistance     offered,  method  of 

representing,  367 
theory  of  Helmholtz,  136 
theory  of  Tscherning,   137 
zonula  affected   in,    138 
Action  of  a  single  muscle,  186 

of  the  double  prism,  230 
Act  of  accommodation,  137 

of  winking,  measurement  of,  210 
Adduction,  definition  of,  179 
Adductive  prism,   280 
Adductors,  lifting  power  of,  200 
After-images,  269 

experiments  with,  273 
Aids  to  calculation  of  torsion,  275 


Alcohol,  5 

Alpha,  angle,  126,  128,  129 
Altitude,  definition  of,  180 
Amphibians,  recti  of,  112 
Anatomy,  reason  for  reviewing,  i 

thorough  study  of,   380 
Angle  alpha,  126,  128,  129 

clinical  value  of,  129 

earliest  method  of  estimating, 
129 

exact  method  of  measuring,  131 

of  Donders,  128 

of  Landolt,   128 

simple  method  of  measuring, 

.J3°. 
size  of,  129 

Angle  delta,  129 

gamma,  128 

refracting,  281 

Animals,  accommodation  in,  140 
Anophoria,  definition  of,  220 
Anterior  lenticonus,  137 

surface  of   lens    in    accommo- 
dation 142 
Apparent  rest,   220 
Arc  of  perimeter,  190 

of  rotation,   204 
Arteria  fossae  sylvii,  109 
Astigmatic  accommodation,  153, 

I54 
contraction   of  ciliary  muscle, 

J53 
Atropin  sulphate.diagnostic  value 

of,  171 

in  full  doses,  159 
in  minimum  doses,   164 
value  of  minimum  doses  of ,  168 
Author's  arrangement  of  Le  Con- 

te's  squares,  354 
ophthalmotrope  of,  183 
optometer  of,   314 
telescope  visuometer  of,  218 
tortometer  of,  355 
tortometer  of.invariousplanes, 

356 

visuometer  of,  216 
Axes,  vertical,  usual  position  of, 

250 
visual,  determination  of  plane, 

349 


448 


Index  of  Subjects 


44Q 


Axes — Continued. 

visual,  determining  static  posi- 
tion, 239 

visual,  usual  position  of,  241 
Axis,  horizontal,  120 

of  horizontal  muscles,   122 

of  oblique  muscles,  122 

of  vertical  muscles,  r  2  2 

optic,  120,  127 

vertical,  120 

visual,  128,  237 
Azimuth,  definition  of,  180 

minus,  definition  of.  180 

plus,  definition  of,  180 

B 
Balance  of  Ocular  muscles,  366 

of  power,  303 
Ball,  plain  rubber,  181  . 
Base  line  measured  by  visuometer, 
3i8 

measurement    of,  215 

table   in   millimeters,    296 
Belladonna,  full  dose  of,  160 
Birds,  muscles  of,  112 
Blank  for  measurement  of  relative 

accommodation,  317 
Bowman,  muscle  of,  58 

C 
Calculation  of  torsion  with  parallel 

axes,  274 

Capsule  of  Tenon,  40 
Cells,  function  of,  98 

in  cortex,  90 
Center  of  motion,  123 

calculation  of,  124 
Centrad,  281 
Chagrin  of  lens,  64 
Check  ligaments,  45 

external,  48 

inferior,   48 

internal,  46 

superior,  49 
Check  ligaments,  surgical  anatomy 

of,  49 
Ciliary  muscle,  anatomy  of,  56 

in  accommodation,  138 

muscles,  239 

Circular  fibers  of  ciliary  muscle,  58 
Circumduction,  definition  of,  179 
Classification  of  tests,  224 
Clear  vision,  nearest  point  of,  155 
Clinical  examinations  more  thor- 
ough 382 

importance  of  false  torsion,  278 

value  of  rapidity  of  lateral  mo- 
tion, 208 


Clinoscope,  another  form  of,  248 

converging,  348 
Clinoscopes,  348 

Stevens',  247 
Cobalt  glass,  234 
domain  as  a  mydriatic,  172 
Comparative  anatomy,  literature 

of ,  1 1 1 

Compound  Maddox  rod,  231 
Connective  tissue  in  orbit,  42 

of  orbit,  40 
Convergence,  292,  300 

appliance  for  measuring  torsion 
with,  348 

definition  of,  280 

degrees  of,  expressed  in  meter 
angles,  295 

relation  to  accommodation,  341 

relation  to  torsion,  348 

relative,  341 

relative,  clinical  importance  of , 

347 

relative,  diagrammatic  repre- 
sentation of,  344 

relative,  how  to  measure,  343 

relative,  measured  for  clinical 
purposes,  346 

relative,  table  of,  344 

test  of  muscle  balance  with,  303 

torsion  with,  artificially  dis- 
turbed, 363 

torsion  within  horizontal  plane, 

•  359 

torsion  with,  object  of,  363 
varying  degrees  of,  309 
with  resistance  offered,  method 

of  representing,  367 
Cornea,  curvature  of,  76 
Corresponding  points  of  retina, 

214 

Corrugator  supercilii,  80 
Cortex,  cells  in,  93 
Cover  test,  235 

Cycloduction,  definition  of,  179 
Cyclophoria,  definition  of,  220 
nature  of,  251 
which  test  is  best,  251 
Cycloplegic,  cocain  as  a,   172 
Cycloplegics,  effect  of  minimum 

doses  of,  178 
effects  of,  154 


D 


Data,  imperfection  of,  in  reference 
to  torsion,  363 


450 


Index  of  Subjects 


Decalcification,  6 
Declination,  definition  of,  180 
Definition  of  anophoria,    220 

of  cyclophoria,   220 

of  esophoria,  220 

of  exophoria,  220 

of  heterophoria,  220 

of  hyperphoria,  220 

of  katophoria,   220 

of  orthophoria,  220 
Definitions,  more  exact  are  neces- 
sary, 381 

Deflection  produced  by  prism,  289 
Deflections,  table  of,  289 
Degrees,    meter  angles  expressed 

in,  292 

Delta,  angle,  129 
Desiderata    for  measurement  of 

relative,  convergence,   342 
Development  of  the  nerves,  116 
Dextral  subductors,  definition  of, 
189 

superductors,  definition  of,  189 
Dextroiuctors,  definition  of,  188 
Diagnostic  value  of  atropin  sul- 
phate, 171 
Dilator  pupillae,  58 
Diplopia,    suppression  of  physio- 
logical, 133 

Diploscope  of  Remy,  235 
Discs,  ophthalmic,  155 

Volkmann,  modifications  of ,  261 
Dissection,   2 

of  human  orbit,  7 

of  the  ophthalmic  ganglion,  107 
Dissections  of  animals,   7 
Donders,  angle  alpha  of,  128 

law,  273 

method,  259 

ophthalmotrope,  183 
Dots,  arrangement  for  testing  het- 

eroohoria,  226 
Drops  differ  in  size,  155 


E 


Effect  of  minimum  doses  of  cyclo- 
plegics,    178 

Effects  of  cycloplegics  and  mydria- 
tics,   154 

Electric-light  attachment  to  peri- 
meter, 192 

Environment,  221 

Equatorial  plane,   120 

Eserin,  full  dose,  174 
minimum  dose,  176 

Esophoria,  definition  of,  220 


Eukinesis,  368,  370,  371 
Examination,  objective  methods 

preferable,  198 
routine  of,  382 
Exophoria,  definition  of,  220 
Experiments  with    after-images, 

273 

Extent  of  field  of    fixation,   196 
External  rectus,  description  of,  22 
Extorsion,  definition  of,  179 
maximum  and  minimum,  262 
physiological,   amount  of,  263 
Eyes,  normal,  visual  axes  in  ap- 
parent rest,  241 


False  torsion,  clinical  importance 
of,  278 

definition  of,   179 

not  true  wheel  motion,  271 
Fascia  orbito-ocularis,  45 

demonstration  of,  46 
Fibers  in  the  brain,  93 
Field  of  fixation,  189 

clinical  value  of,  197 

extent  of,  196 
Field,  the  motor,  189 
Fifth  nerve,  103 

Foetal  development    of  the  mus- 
cles, 114 
Formalin,    6 
Fourth  nerve,   102 
Full  dose,  definition  of,  160 

of  belladonna,    160 

of  eserin,  174 

of  homatropin,  169 
Full  doses  of  atropin  sulphate,  159 
Function  of  groups  of  cells,  98 
Fusion  power,  how  measured,  298 

maximum,  300,  303 

minimum,  299,  300 

what  is  the,  297 


Gamma,  angle,  128 
Ganglion,  ophthalmic,  107 
Gasserian  ganglion.  103 
Gauge  for  distance  between  pupils 

2IS 

General  strength  and  ocular  mus- 
cles, 373 

Geometry  of  the  globe,    119 

Glass,  cobalt,  234 


Index  of  Subjects 


Globe,  geometry  of,  119 
measurements  of ,  1^2 
Gracillimus  muscle,  52 
Gun-sight    attachment    to    peri- 
meter, 191 


H 


Hardening  and  fixing  fluids,  5 
Head-rest,  222 

Helmholtz's  theory  of  accommo- 
dation,  136 
Heredity,  influence  of,  on  muscles, 

54 

Hering's  method,  257 
Hess'    method,    relative    accom- 
modation, 330 
Heterophoria,  definition  of,  220 

nature  of,  219 
Homatropin,  effect  of  full  dose,  169 

hydrobromate,  diagnostic  value 

of,  171 
Horizontal  axis,  120 

muscles,  axis  of,  122 

plane,   119 

Homer,  muscle  of,  51 
Hyperphoria,  definition  of,  220 
Hypophoria,  definition  of,  220 


Illumination,  intensity  of,  149 
Image  falls  unchanged  on  mac- 
ula, 246 

retinal,  clear  but  displaced,  245 
Imperfections  of  the  media,  77 
Inferior  oblique,  description  of,  30 

rectus,  description  of,  26 
Inflation  of  the  globe,  10 
Injection  fluids,  4 
Insertion  of  the  recti,  37 

physiological,  186 
Insertions,  lateral  secondary,  34 

ocular,  secondary,  34 

orbital,  secondary,  35 

primary,  32 

primary,  as  a  whole,  36 

secondary,  34 

Inspiration  affects  size  of  pupil,  149 
Internal  check  ligament,  46 

rectus,  description  of,  17 
Intorsion,  definition  of,  179 

maximum  and  minimum,  262 

physiological,  amount  of,  263 
Intraocular  muscles,   56 
Iris  in  accommodation,  137 


Javal  ophthalmometer,  modifica- 
tion of,  69 

K 

Katophoria,  definition  of,  220 
Kinetograms,  207 
Knapp's  ophthalmotrope,  183 
Knowledge  like  a  pyramid,  118 


Landolt,  angle  alpha  of,  128 
Landolt's  ophthalmotrope,   181 
Lateral  cells  of  nucleus,  90 
Lateral  motion,  203 

clinical  value  of  rapidity  of,  208 

rapidity  of,  202 

Lateral  secondary  insertions,  34 
Lateralis, rectus, description  of,  22 
Law,  Bonders,  273 

Listing's,  274 

Le    Conte's   squares,  author's  ar- 
rangement of,  354 
Lens  affects  accommodation,  145 

chagrin  of,  64 

faces  outward,  68 

falling  of,  138 

how  to  determine  position  of  ,67 

position  of,  66 

stenopaic,  233 

structure  of,  62 

tips  vertically,  68 
Lenses,  prismatic  effect  produced 

by  decentering,  290 
Lenticonus,  anterior,  137,  139 
Lasval  subductors,  definition,  189 

superductors,  definition  of,  189 
Levator  palpebrae,  16 

description  of,  15 
Laevoductors,  definition  of,  188 
Lifting  power  of  th e  adductors ,  2  oo 
Ligamentum  suspensorium  oculi, 

49 
Lines  near  center  of  lens,  64 

near  periphery  of  lens,  64 

of  origin,  1 1 
Listing's  Law,  274 

Plane,  121 
Lower  vertebrates,  no 

M 

Maddox  rod,  compound,  231 
measurements  of  torsion  with, 

259 
simple  form,  230 


452 


Index  of  Subjects 


Measure  of  size  of  pupil,  157 
Measurement  of  interocular  base 
line,  215 

of  pupillary  reaction,  148 

of  relative  accommodation,  313 
Measurements,  clinical  value  of, 
concerning  torsion,  365 

of  globe,  122 

Measuring  field  of  fixation,  189 
Media,  imperfections  of,  77 
Meter  angle,  definition  of,  292 
Meter  angles,  degrees  of  conver- 
gence expressed  in,  295 

expressed  in   degrees,  292 
Method,  Donders',  259 

Hering's,   257 

Microscope,  ophthalmic,  150 
Minimum  dose,  definition  of,  160 

dose  of  atropin  sulphate,  1 64 

doses  of  atropin  sulphate,  prac- 
tical value  of,  1 68 

dose  of  eserin,  clinical  value  of, 
177 

dose  of  eserin  for  diagnostic  pur- 
poses, 176 

Minus  azimuth,  definition  of,  180 
Monocular  position  of  rest,  134, 
Motion,  center  of,    123 
Motor  field,   189 

oculi,  origin  of,  90 
Movement  of  eye  while  reading, 

208 

Movements,  associated, classifica- 
tion of,  255 

definition  and  mechanism  0^253 

first  group  of,  256 
Mueller's  rule,   214 
Muller,  muscle  of,  58 
Muscle  balance,  370 

and   orthophoria,  371 

diagrammatic  illustration  of,  3  68 

employment  affects,  370 

influenced  by  age,  370 

in  general  health,  370 

test  of,  with  convergence,  303 
Muscle  of  Korner,  51 
Muscles,  ciliary,  239 

opposing  action  of,  187 

recti,  240 
Muscle  tone,  374 
Muscular  strength,  Sargent's  test, 

373 

Mydriasis,  definition  of,  165 
Mydriatic,  cocain  as  a,  172 
Myotirs,  effect  of  minimum  doses 

of,  178 


N 


Nature  of  heterophoria,  219 
Nearest  point  of  clear  vision.  155 
Nerves,  development  of  the,  116 
New    York    State  School  for  the 
Blind,   examination  at,  134 
Normal  pupil,  152 
Nucleus  in  pons,  90 
Numbering  prisms,  281 


O 


Objective    methods  of  examina- 
tion, why  preferable,  198 
Oblique, inferior,description  of,  30 
muscles,  axis  of,  122 
superior,  description  of,  27 
Occipito-frontalis  and  trapezius,83 
Ocular    muscles    and    general 
strength,  relation  between, 

373 

muscles  and  other  muscles,  373 

secondary  insertions,  34 
Oculo-facial  group,  101 
One  eye  in  action,  136 
Ophthalmic  discs,   155 

ganglion,    107 

microscope,  150 
Ophthalmological  prisms,   280 
Ophthalmometer,  Javal-Schiotz, 

124 
Ophthalmophacometer  of  Tscher- 

ning,  72 
Ophthalmotrope,   Bonders',    183 

Knapp's,  183 

Landolt's,  181 

o'f  author,  183 
Ophthalmotropes,  180 
Opposing  action  of  muscles,  187 
Optic  axis,  120,  127 
Optometer,  method  of  measuring 
relative  accommodation,  317 

of  author,  314 
Orbicularis  palpebrarum,  83 
Orbital  secondary  insertions,  35 
Orthophoria  and  muscle  balance, 

37i 
definition  of,  220 


Palpebrae,  levator,  16 
Parallax  test,  234 
Pereles'method  of  relative  accom- 
modation, 329 


Index  of  Subjects 


453 


Perimeter,  arc  of,  190, 

description  of,  190 

electric-light  attachment,  192 

gun-sight  attachment  to,  191 

telescope  attachment  to,  192 
Phorometer,  222,  223 
Photograms,  207 
Physiological  insertion,  186 
Plan  of  study  of  physiology,  118 
Plane,  equatorial,  120 

Listing's,    121 

horizontal,  119 

vertical,  120 

Plus  azimuth,  definition  of,  180 
Position,  of  rest,  240 

of  rest,  monocular,  134 

primary,    121 

Positions  of  retinal  images  in  eso- 
phpria,  225 

of  retinal  images  in  orthophoria, 

225 

Posterior  surface  of  lens  in  accom- 
modation, 143 
Precautions    necessary    with    all 

tests,  221 
Pre-eminent  muscular  functions, 

187 

Preserving  fluids,  3 
Primary  insertions,  13 

insertions  as  a  whole,  36 

insertions,  measurements  of ,  32 

position,  121 
Prism,  abductive,  281 

action  of  double,  230 

actual  deflection  produced  by, 

289  adductive,  280 
Prismatic  effect  produced  by  de- 
centering  lenses,  290 

diopter,  282 
Prisms,  in  what  form  arranged,  282 

numbering  of,  281 

ophthalmological,  280 
Provisional  diagnosis  necessary, 

382 
Pupil,  in  accommodation,  137 

measure  of  size,  157 

size  of,  148,  149 

normal,  152 

unnaturally  large ,   152 

unnaturally  small,  152 
Pupillary    reaction,    measurement 
of,  148 

reaction,  types  of,  152 
Pyramidalis  nasi,  80 

R 
Radial  portion  of  ciliary  muscle,  57 


Range  of  accommodation,  144 
modified   by  age,  145 
Rapidity  of  lateral  motion,  202 
Reading,  movement  of  the  eye 

while,  208 

Recapitulation,  376 
Recording    attachments   of   the 

muscles,  33 
Recti,  insertion  of,  37 

muscles,  240 

Rectus,  external, description  of, 2 2 
external,  external  surface,  23 
inferior,  description  of,  26 
internal,  description  of,  17 
lateralis,  description  of,  22 
superior,  description  of,  24 
Reflecting  stereoscope,  305 
Refracting  angle,  281 
Refractive  media,   imperfections 

of,  79 
Relative  convergence,  blanks  for 

recording,  342 
clinical  importance  of,  347 
desiderata  for  measurement  of, 

.342 
diagrammatic  representation  of 

344 

how  to  measure,  343 
Reptiles,  recti  of,  112 
Respiration  affects  size  of  pupil, 

149 

Rest,  apparent,  220 
both  eyes  at,  214 
Resultant  movement,  186 
Retina,  condition  of,    influences, 

static  position,  240 
corresponding  points  of,    214 
Retinal  image  clear  but  displaced, 

245 

Retinal    images,   positions  of,  in 
esophoria,  225 

positions  of,  in  orthophoria, 22 5 
Rodents,  orbits  of,    113 
Rod  of  Maddox,  simple  form,  230 
Rotation,  arc  of,  204 

in  certain  directions,  188 
Rubber  ball,  181 

bands  to  mark  planes,  33 
Rule,  Mueller's,  214 


Secondary  insertions,  13,  34 

Sixth  nerve,  105 

Size  of  pupil,  148,  149 

Sound  produced  by  eye  muscles, 


454 


Index  ot  Subjects 


Spots  in  lens,   65 
Static  position  of  visual  axes,  con- 
clusions  regarding,  242 
Stenopaic   disc,    65 

lens,  233 

lens,  action  of,  234 
Stereoscope  cards,  306 

refracting,    306 

refracting,  clinical  value  of,  306 
Stereoscopes,  305 
Stevens'  tropometer,   description 

of,  193 
Strength,  tensile,  of  the  recti,  201 

test,  Sargent,  373 
Subduction,  definition  of,  jyg 
Subductors,  dextral,  definition  of, 
189 

loeval,   definition  of,    189 
Sublimate  solutions,    5 
Subsidiary  muscular  functions,  187 
Superduction,   definition  of,    179 

dextral,  definition  of,  189 

lasval,  definition  of,   189 
Superior  oblique,  description  of ,  27 

rectus,  description  of,  24 

rectus,  ocular  surface,  25 
Supernumerary  muscles,  51 
Suppression  of   diplopia  physio- 
logical, 133 
Sympathetic,  branches  of,  106 


Table,  base  line  in  millimeters,  296 

of  deflections,  289 

of  torsion  in  horizontal  plane,  360 

prism,    282 

relative  accommodation,   319 

torsion    above    or   below  hori- 
zontal plane,  361 
Tangent  scale,  195 
Telescope  attachment  to  peri- 
meter, 192 

visuometer,  author,  218 
Tenon,  capsule  of,  40 
Tensile  strength  of  recti,  201 
Test,  cover,  235 

light,  222 

parallax,  234 

types,  construction  of,  132 

types  for  near  point,   132 
Testing,  desiderata  for  relative  tor- 
sion, 362 

scales,   282 
Tests,  classification  of,  224 

precautions  necessary  with,  221 

third  group  of,  234 
Third  nerve,  origin  of,  90 


Tonus  muscularis,  374 
Torsion,  aids  to  calculation  of  ,275 
appliances  for  measuring,  351, 

352 
appliances    for  measuring,   Le 

Conte,  352 

clinical  importance  of ,  263 
definition  of,  179 
false,   207 

how  to  plot  in  given  plane,  361 
measurements  of  difficult,  350 
measurements  of  with  Maddox 

rod,    259 
relative,   362 
relative,  desiderata  for  testing, 

362 

true,  244 

true,  possible  with  one  eye,  211 
with  convergence,   348 
with  convergence  above  or  be- 
low the  horizontal  plan,  360 
with  convergence,     appliances 

tor  measuring,  348 
with     convergence    artificially 

disturbed,  363 
with  convergence  in  horizontal 

plane,    359 

with  convergence,  object  of, 3  63 
with  parallel  axes,  calculation 

of,  274 
with  resistance  offered,  method 

of  representing,  367 
Tortometer,  348 
of  author,  355 
scale  on,  358 
Transversus  muscle,  52 
Trapezius,  and  occipito-frontalis, 

.    83 
Tngeminus,  103 

Trochlearis,    102 

Tropometer,  Stevens',  description 

of,    193 

True  torsion,  definition  of,  179 
torsion  possible  with  one  eye, 

211 

Tscherning's  theory  of  accommo- 
dation, 137 
Types  of  pupillary  reaction,  152 

U 

Uniformity  in  methods,  need  of' 
38i 

V 

Vertebrates,  lower,   no 
Vertical,  axes,   tests  to  determine 

position  of,  244 
axes,  usual  position  of,  250 


Index  of  Subjects 


455 


Vertical — Continued. 

axis,  1 20 

muscles,  axis  of,  122 
plane,  120 
Vision,  comfortable,  factors  in  pro- 
duction of,  366 
Visual     acuity  and  action   of  eye, 

!3i 

axes,  conclusions  regarding  sta- 
tic position,  242 
axes,  determining  static  position 

of,  239 
axis,  128 
axis,  methods  for  determining 

latent  position,  237 
camera,  134 
Visuometer,  interocular  base  line 

measured  by,  318 
of  author,  216 
of  Hess,  217 

Vitreous  seldom  clear,  77 
Volkmann  discs,  modifications  of, 
261 


W 


Wheel  motion,  false  torsion,   271 
Winking,  measuremen.  of,  210 


Young  subjects  for  measurement 
of  torsion,  350 


Zinn,  suspensory  ligament  of.  60 

zonula  of,  58 

Zonula,  how  affected  in  accommo- 
dation, 138 

of  Zinn  58 

relaxes,  141 

Zinii,  60 


BIOGRAPHIC   NOTES 

OF    A    FEW     EMINENT    STUDENTS    OF    THE   OCULAR  MUSCLES. 

The  beginner  in  any  branch  of  science  too  often  forgets  that 
he  is  the  ' '  heir  of  all  the  ages,"  and  that  any  text -book  is  largely 
a  digest  of  the  work  of  many  former  laborers  in  the  same  field. 
But  after  one  has  confined  his  studies  for  some  time  to  a 
subject  which  is  apparently  small,  and,  in  doing  so,  found 
frequent  evidences  of  the  patient  investigation  of  others,  such 
a  student  often  desires  some  glimpse  of  the  personality  of  the 
men  who  have  already  worked  out  many  of  his  problems,  and 
to  whom  he  owes  a  debt  of  gratitude.  These  short  biographic 
notes  are  therefore  added  with  the  pictures  of  a  few  men  who 
have  contributed  especially  to  our  knowledge  of  the  anatomy 
and  physiology  of  the  ocular  muscles. 


457 


BREWSTER,  SIR  DAVID.  Born  December  n,  1781.  Educated  at  the  University 
of  Edinburgh.  Was  ordained  a  clergyman,  but  gave  up  the  ministry  and 
devoted  his  life  to  the  study  of  optics.  Described  the  stereoscope  in  1849. 
Between  1806  and  1868  published  over  three  hundred  contributions  to 
scientific  subjects  relating  principally  to  optics.  Died  February  10,  1868. 


459 


HELMHOI.TZ,  HERMANN  LUDWIG  FERDINAND  v.  One  of  the  founders  of 
modern  ophthalmology.  Born  August,  1821.  1849,  Professor  of  Physiology 
and  General  Pathology,  Konigsberg  ;  1858,  Professor  of  Physiology,  Heidel- 
berg; 1871,  Professor  of  Physics,  Berlin.  Invented  the  ophthalmoscope  and 
made  other  invaluable  contributions  to  ophthalmology.  His  principal 
additions  to  our  knowledge  of  the  muscles  are  in  the  Handbook  of  Physio- 
logical Optics  (1856)  and  in  his  frequent  articles,  especially  in  Graefes 
Archives.  Died  September  8,  1894. 


461 


BONDERS,  FRANS  CORNELIS.  Born  May  27,  1818.  1848,  Professor  of  Physi- 
ology, Utrecht;  1855,  one  of  the  editors  of  Graefe's  Archives.  In  these 
archives  there  appeared  frequent  articles  on  accommodation,  muscular 
movements,  torsion,  etc.  In  1863  he  published  Pathology  of  Strabistmts, 
and  in  1864  his  epoch  making  work  on  the  Anomalies  of  Refraction  and 
Accommodation.  He  made  valuable  contributions  to  our  knowledge  of 
the  action  of  mydriatics  and  myotics  and  furnished  many  other  data  con- 
cerning the  ocular  muscles.  Died  March  24,  1889. 


463 


HERING,  EWALD.  Born  1834.  Followed  Purkinje  at  the  University  of  Prag  in 
1870,  as  Professor  of  Physiology  and  Medical  Physics.  In  1895  occupied 
the  corresponding  chair  at  Leipzig.  Voluminous  writer  in  Poggendorff's 
Annals  and  in  Grae/e's  Archives.  Studied  particularly  binocular  vision 
and  the  associated  movements  of  the  eye. 


465 


NAGEL,  ALBRECHT.  Born  in  1833111  Danzig.  Educated  at  Konigsberg.  Was 
a  student  of  Albrecht  von  Graefe.  Became  Professor  of  Ophthalmol- 
ogy at  Tubingen  in  1874  and  died  there  in  1895.  His  contributions  to  this 
subject  were  in  showing  the  relation  between  accommodation  and  converg- 
ence as  expressed  by  the  meter  angle.  He  also  established  the  invaluable 
Jahres-Berichti  still  published  under  his  name. 


467 


LE  CONTE,  JOSEPH.  Born  in  1823  in  Liberty  Co.,  Georgia.  1845,  College  of 
Physicians  and  Surgeons,  N.  Y.  Served  in  Confederate  Army  later. 
Professor  of  Natural  History,  University  of  California.  Died  1901. 
Published  careful  studies  of  monocular  and  binocular  vision,  and  inves- 
tigations of  torsion  with  convergence. 


469 


HESS,  CARL.  Born  March  7th,  1863.  Graduated  in  Leipzig,  1886.  1896  Pro- 
fessor of  Ophthalmology  in  Marburg.  Made  investigations  in  physiological 
optics,  especially  of  the  act  of  accommodation.  Frequent  articles  in  Graefes 
Archives.  Recipient  of  the  Graefe  prize  in  1900  particularly  for  his  articles 
on  accommodation.  At  present  Professor  of  Ophthalmology  at  Wurzburg. 


47i 


TSCHERNING,  MARIUS  HANS  ERIK.  Born,  1854,  on  the  Island  of  Fionie,  Den- 
mark. 1884,  adjunct  director  with  Javal  of  the  laboratory  of  ophthalmology 
at  the  Sorbonne.  Has  made  extensive  researches  concerning  the  mechan- 
ism of  accommodation  and  in  other  departments  of  physiological  optics. 
Author  of  work  on  that  subject  (English  translation).  Contributed  articles 
on  dioptrics  and  on  the  ocular  movements  to  the  third  volume  of  the  French 
Encyclopedia  of  Ophthalmology. 


467 


TunCli 


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*"~V~ **  ( 


